The heterogeneous selective liquid-phase oxidation of aldonic acids to the corresponding 2-ketoacids over a lead- or bismuth-modified platinum catalyst was reinvestigated. The influence of the pH (pH range 2-9) on the nature and the stability of the chelate complex formed between aldonic acid and the promotor (Pb, Bi), which is assumed to govern the selectivity, was found to be important for the performance of the catalyst. The non-selective complexation in alkaline medium of the promotor with 2-keto-aldonic acid was assumed to be the cause of the decrease of the selectivity at higher conversion. In weakly acidic medium (pH < 6) when poisoning of the catalyst was avoided over 98% selectivity at over 98% conversion was achieved.