Microbiological processes play a major role in biogeochemical cycling of organic matter in the ocean. However, little is known about the direct effects of ocean acidification on heterotrophic activity and, hence organic matter cycling through the microbial loop. To elucidate whether future ocean acidification will directly affect microbial organic matter degradation, a study was conducted examining the impact of decreasing pH on bacterial hydrolytic enzyme activities. Different acid treatments were used to induce different pCO2 concentrations in offshore mesocosms during a series of short time experiments. The study was carried out in the Baltic Sea in July 2007 in the frame of the SOPRAN (Surface Ocean PRocesses in the ANthropocene) project. Based on kinetic measurements, maximum turn-over rates (Vmax) and half-saturation constant (Km) were calculated for glucosidases, peptidases and phosphatase. A decrease in pH led to a significant increase in Vmax of carbohydrates accompanied by a decrease of peptide degradation. Possible implications of this short time changes in heterotrophic organic matter turnover due to acidification will be discussed.