The reduction reaction of dissolved oxygen (DO) by hydrazine was investigated on activated carbon fiber (ACF) and Pt/ACF catalysts using a batch reactor with an external circulating loop. The ACF itself showed catalytic activity and this was further improved by supporting platinum on the ACF. While the reaction order of DO was obtained as 0.5 on both the ACF catalyst and the Pt/ACF catalyst, that of hydrazine was observed to be 1.0 on the ACF catalyst and 0.25 on the Pt/ACF catalyst. The catalytic role of platinum is ascribed to its acceleration of hydrazine decomposition, based on electric potential and current measurements, as well as the kinetic study. The activation of oxygen may to be attributed either to ACF or platinum.