The rate at which H2O2 becomes available during glycollate oxidation for further oxidation reactions, especially that of glyoxylate to formate and CO2, in peroxisomes from spinach-beet (Beta vulgaris L., var. vulgaris) leaves has been determined by measuring O2 uptake in the presence and absence of added catalase. The rates observed under air and pure O2 were sufficient to account for the (14)CO2 released from [l-(14)C]glycollate under these conditions; the two reactions showed similar characteristics. In the course of the reaction, a fall in catalase activity was observed concomitant with an increase in (14)CO2 release. There is no evidence that catalase was disproportionately lost from the peroxisomes during isolation, and it is argued that the CO2 release observed contributes to the photorespiratory CO2 loss in intact leaves.