Sustained oscillations of catalyst temperature have been observed in NH3 oxidation on Pt wires and foils in a 1-atm flow reactor. Simple and complex oscillations with periods from < 1 sec to several minutes were obtained for gas compositions between 20 and 40% NH3 in air. These depend sensitively on gas composition, flow velocity, and geometry. Oscillatory behavior is quite stable over short times, but over several hours patterns evolve slowly. Upon perturbation of composition, the average temperature is observed to attain a new steady state much more rapidly than the oscillation pattern. This is a complex reaction system in that several reactions occur, heat and mass transfer are important, and natural convection and homogeneous reaction are significant under some conditions. However, results were quite reproducible between specimens, and overall behavior exhibits clear evidence of the importance of surface processes in oscillations.