Field experiments were conducted to evaluate the effects of soil properties, such as the cation exchange capacity and mineral content, on pH, soluble ion concentrations, and electrical conductivity during electroosmosis in a silty clay soil. The soil is composed mainly of quartz (60%) and smectite/illite (20--29%). The measured cation exchange capacity of the test site soil ranged from 11 to 22 cmol kg-1, which was consistent with the contribution from the clay contents. The exchangeable cations and reserve acidity measurements suggest that the majority of protons that were generated at the anode by electrolysis not only exchanged the base cations but also were adsorbed on the soil minerals. Protons also reacted with the soil minerals and caused the release of dissolved Al3+, which was then exchanged and adsorbed by the soil minerals. The exchange and adsorption of proton and Al3+ resulted in the decrease of the exchangeable base cations and ion concentrations of the pore fluid. Sorption (exchange and adsorption) and reaction significantly retarded the transport of protons. The apparent retardation factor for protons calculated from the movement of the pH front was of the order of 1000, whereas retardation factors calculated by assuming a Langmuir isotherm ranged from 30 to 6700 in the pH range of 3.6--4.8. The electrical conductivity of the soil also was influenced and dominated by the sorption and reaction of protons. In the region close to the anode where protons were sorbed, the electrical conductivity at the end of the test decreased to 0.37 of the initial value despite the fact that the proton concentration in pore fluid increased by 1--2 orders of magnitude. The results of this study show that soil properties, such as cation exchange capacity and mineral content, have a pronounced effect on solution chemistry and hence the utility of electroosmosis. ¿ 1999 American Geophysical Union