- Abstract Organic compounds derived from agricultural, industrial and domestic waste frequently contaminate soil and ground water. Any attempt to estimate the effect of these organic acids on water quality must include the various interactions between the organic material and the minerals of the host rock and soil. In the present paper, we study the effect of kaolinite surfaces on the degradation rate of oxalate in the presence of microorganisms. The goals are to show that the rate of microbial degradation of organic acids is enhanced in the presence of mineral surface and to evaluate the relative importance of oxalate consumption by microorganism activity vs. (a-biotic) surface catalyzed chemical degradation. We examine both the effect of presence of kaolinite on the rate of oxalate degradation using a flow-through reactor, and oxalate adsorption on the kaolinite surface using a batch reactor. Experiments were conducted at 25°C and 50°C and pH 3. The measurement of the adsorption of organic anions on mineral surfaces is based on change in the organic anion concentration in solution. However, part of this change in concentration may result from degradation of the organic anions, which may be enhanced by microorganism activity. The change of oxalate concentration on the kaolinite surface as a function of oxalate concentration in solution is described by a general adsorption isotherm: C s =8×10 −7 20C Ox 0.4 1+20C Ox 0.4 . The decomposition rates in the flow-through experiments ranged from 2×10 −11 to 2×10 −10 mol s −1 , and depended on the amount of kaolinite present in the cell. Adding an antibiotic mixture to the input solution or increasing the temperature to 50°C halted the oxalate decomposition. Therefore, we conclude that oxalate decomposition at 25°C and pH 3 is controlled by microorganism activity and not by chemical (a-biotic) processes. By adding the same antibiotic mixture to the adsorption experiments, we show that the measurement of oxalate adsorption on the kaolinite surface is not affected by oxalate biodegradation.