This paper presents the results of a systematic analysis aimed at establishing whether acidic pore fluids can affect the properties of natural soils, in particular their compressibility. Marine deposits with different mineral compositions and undisturbed soil structure were collected for this research from three coastal areas in Japan. Pleistocene clays from the Osaka and Ariake Bays were obtained from boreholes at a depth of more than 10 m, whereas the Kawasaki mud, a relatively young deposit of Holocene, was dredged from the bed of the Tokyo Bay. Soil specimens were placed in special containers, which were designed to reproduce the process of long-term soil- water-chemical interaction, and leached with solutions of sulfuric acid for different periods of time, ranging from 1 to 9 months. At the end of each time interval, standard compression tests were performed to study the behavior of soil in an acidic environment. It was found that clay mineralogy and soil structure had a significant effect on the compressibility of clays at low pH. In the case of the Osaka and Ariake clays, the compressibility significantly increased with a decrease in pH values, a finding that was primarily attributed to changes in the soil's structure. In contrast, the effect of acidic leaching on the properties of Kawasaki mud was observed to be the opposite. Laboratory data showed that in acidic medium the compressibility of soil decreased presumably due to the collapse of the diffuse double layer.