Abstract: The diluent solution of glyoxylic acid and oxalic acid was used as the separation object.TOA (in n - octanol) was used as the solvent. The extraction characteristics of single acid and two-acid solution were studied, and the influence of pH and the concentration of complexing agent on the distribution coefficients were measured. The suitable mathematic models were proposed to describe the relationship between TOA and carboxylic acid based on assumed extraction reaction in organic phase. The experimental results showed that the ratio of oxalic acid to TOA was 1∶1 and 1∶2 in the organic phase. The distribution coefficient of oxalic acid was larger than that of glyoxylic acid, and both of them decreased with the pH. The selectivity of oxalic acid to glyoxylic acid also decreased with pH. In the extent of this experiment, the distribution coefficients for each acid and the selectivity increased with the concentration of complexing agent , and a higher selectivity and the distribution coefficients could be obtained at higher pH for a larger concentration of complexing agent. The calculated results were close to the experimental data based on the models using the parameters K 11 and K 12 from the single acid experiment and m A and m B from the two-acid experiment in the presence of 100% n -octanol.