The effect of oxygen on the Ce4+-malonic acid reaction was studied in a semibatch reactor. That effect is important for the Belousov-Zhabotinsky (BZ) chemical oscillator. The Ce4+ reagent inflow and consequently the rate of the reaction itself was controlled by a peristaltic pump. The reaction products were analyzed by HPLC. With this technique two major oxidation pathways were identified. One is significant at high Ce4+ inflow rates only; the product of this channel is tartronic acid. The other pathway leading to oxalic acid is active at all flow rates but dominant when the feed is slow. A great part of oxalic acid is oxidized further to carbon dioxide and water. A reaction mechanism compatible with these findings is presented. A key step of this mechanism is the fate of the peroxymalonyl radical which is the first intermediate for both channels. It is proposed that at high Ce4+ concentrations a fast reaction of this intermediate with Ce4+ leads to tartronic acid. At low Ce4+ concentrations. however, the peroxymalonyl radical has a longer lifetime to decarboxylate before reacting with a second Ce4+ and giving oxalic acid this way. Two mechanistic schemes proposed fur this low Ce4+ channel were tested with further HPLC and kinetic experiments. From the high and low Ce4+ channels it is only the low one which plays a significant role in oxygen-perturbed BZ systems, The effect of that channel and its intermediates on BZ oscillators is discussed briefly.