The catalytic decomposition of hydrogen peroxide (H₂O₂) into water (H₂O) and oxygen (O₂) [H₂O₂ → H₂O + ½ O₂] is notoriously susceptible to thermal runaway (Heat of Reaction: -98 kJ/mol). However, liquid H₂O₂ has high oxygen density making it a particularly attractive oxidant for applications where air-independence and space limitations elicit additional power supply design challenges. Microchemical technologies have intrinsically higher surface-to-volume ratios which facilitate heat and mass transfer, and thus a mechanism to harness the benefits of H₂O₂ decomposition as an oxidant source while reducing the risk of uncontrollable heat rise. This study investigated two small-scale reactors to illustrate thermal management and oxygen production feasibility during the multiphase decomposition of H₂O₂ as an oxidant for an unmanned undersea vehicle (UUV) solid oxide fuel cell (SOFC). A miniature tubular, catalyst-coated (TCC) reactor model with channel radius of 1.5 mm and a microscale packed bed (MPB) reactor model with channel radius 0.5 mm were simulated in the finite element modeling program COMSOL. The properties of the multiphase product stream were described by weighted averages of the components in the stream. This approach assumed that the stream maintained a well-mixed, dispersed phase. Heat generated during simulated decomposition in the convectively cooled reactor channels caused temperature rises greater than 400 K (neglecting the energy required to evaporate water). Although small-scale channel geometry alone failed to prevent significant heat rise, the extension of surface areas around the reactor channel improved passive cooling resulting in simulated temperature rises less than 24 K (TCC) and 7 K (MPB) and experimentally observed temperature rises less than 56 K (TCC) and 4 K (MPB). Pulsating oxygen production, consistent with two-phase slug flow, was observed in both reactors over a range of flow rates. Although slug flow was not accounted for in the model, the time-averaged empirical behavior compared well to the simulated behavior for the MPB. Thermal management of the MPB was found to be superb and clearly superior to that of the TCC. Overall these results showed the feasibility of improved thermal control and oxygen generation during H₂O₂ decomposition with a microchemical approach relative to conventionally sized reactors.