This paper seeks to investigate the effect of acidic contamination on the stress-strain characteristics of two natural soils with different mineral compositions: Kawasaki mud that contains montmorillonite and Yurakucho silt that has kaolinite as the dominant clay mineral. To reproduce the process of soil contamination in the laboratory, a special container was designed in which soil samples were leached with solutions of sulfuric acid for a period of almost 300 days. The laboratory investigation included a series of undrained triaxial compression tests on the leached samples. The obtained results indicate that the acidic contamination had a strong influence on the strength characteristics of the soils, and that the mineralogy of the clay fraction as well as the concentration of acid in the pore fluid significantly influenced the stress-strain behavior of the studied soils. It was found that for the Kawasaki mud, a decrease in pH from 7.3 to 6.0 led to an increase in soil strength. For the Yurakucho silt, a small decrease in pH (pH=6.0) also brought about a slight increase in soil strength. However, a further decrease in pH (pH=4.0) caused a significant reduction in the soil strength. An attempt was made to provide a rational explanation for the observed behavior of the soils based on the fundamentals of colloid chemistry and the available literature.