Anion-exchange resins, a weakly basic polystyrene-polyamine type and a macro-reticular type, IR-45 and IRA-93, respectively, were shown to significantly increase yields of acidic products in co-oxidation systems. p-Toluic, 2,3-dihydroxy-p-toluic, and alpha,alpha-cis,cis dimethylmuconic acids, resulting from the oxidation of p-xylene by three cultures of Nocardia, accumulated on the resin in shaken flasks or agar plates during the cultivation. Final product concentration increased with increasing resin concentration. Mineral balances were not affected if the resin was properly conditioned before use.