It is of common consensus in the ocean acidification community that the increase of atmospheric CO2 is the main driving force of the downwards pH trends in the worlds oceans. In the stations surrounding Sweden, that is most probably the main underlying factor as well, however the rate of change differs from the oceanic rates and there are different rates of change at different depths and different seasons. To investigate further, four monitoring stations with long time series of pH data in the Kattegat and the Baltic Proper have been analysed both for trends and what the main drivers of the change of pH values for those stations could be. Besides a linear trend analysis, a non parametric trend analysis has been applied to the pH data sets. It appears that the carbonate system generally works in the surface layer where the biologic processes are most active, reducing or prohibiting the decline of pH in most of the evaluated stations. It also seems like the downward trends of pH in most of the remaining water masses are influenced and accelerated by oxygen deficiency and eutrophicated water masses. A multivariate analysis was then performed to see what or what combination of parameters influence the change of the pH values the most. The results from the analysis were either significant or not significant, indicating either more trustworthy or not as trustworthy results. A result showing high correlation for a parameter or a set of parameters that influence pH, in combination with being significant, was then an indication of a trustworthy result. Several parameters were included in this analysis, however some key parameters that perhaps influence the changes of the pH values the most may have been missed due to the lack of available data or knowledge or included in the analysis, but in a wrong way. What this study was able to do, was to use the available parameters at hand and make assumptions on how to prepare the data to be able to combine it with the pH data. The results can give an indication as to how much the parameters influence the pH values out of the included parameters, in the manner they were included. Of all the parameters included in the analysis, O2, O2 saturation, PO4 and DIN were the main parameters influencing the pH values. When looking at what single parameter influence pH the most or the least of the included parameters, a table was put together to display what parameters were ranked to be most important and then second most important and so on to the least important parameter. For all stations, all seasons and all depths, there was a slight tendency for the parameters chl-a, atmospheric CO2, North Atlantic Oscillation Index, precipitation pH, river pH and river alkalinity to be ranked the least important. DIN seemed to be more important at the surface layers than at the bottom layers. Salinity and alkalinity seemed to be more important in the bottom layers than in the surface layers. At all depths, O2, O2 saturation, PO4 and SiO4 seemed to be of higher importance. Another interesting feature was that O2 seemed to be of importance throughout all depths except for the 10-20 meters depth, probably due to high variability at that depth. SiO4 seemed to be more important at the Kattegat station than at the other stations. Chl-a did not seem to be important. Since biological activity should have a large impact on pH, chl-a as included in the analysis, was not a good choice as a representative of the biological activity. O2 and O2 saturation were very much influencing the pH patterns. Perhaps in the top layers, they were better representatives for the biological activity in this analysis. It is also interesting to see the lack of importance of the atmospheric CO2. However, when performing trend analysis, not many pH trends were present at the surface (probably due to the biological and of course chemical/physical processes), opening up for O2, O2 saturation and nutrients to be the dominant parameters. In the report, the monitoring need of acidification parameters from a modelling point of view was addressed. The model validation would be very much improved if the concentrations of organic matter could be validated. Today only measurements of total nitrogen and phosphorus and dissolved inorganic nutrients are available. Including standard observations of particulate organic matter (PON, POP and POC) as well as dissolved organic matter (DON, DOP and DOC) would much improve the possibility to further develop the biogeochemical models. Another recommendation is to do a separate investigation based on the results from the coupled oceanographic and biogeochemical model RCO-SCOBI to recommend possible new stations that are important and not yet covered by the present sampling strategy. To calculate and model the saturation state over depth of calcite and aragonite, of high importance for calcifying organisms, the ions CO32- and Ca2+ need to be determined. Either CO32- directly could be measured, or pCO2 and CT (total carbon) could be measured, calculating the desired ion. Further more, the ion Ca2+ could be directly measured, or if not the highest accuracy is needed, estimations could be made from Ca/salinity relationships.