Catalytic combustion of hydrogen in a platinum microtube, or subquenching diameter tube, is studied via theoretical analysis, experiments, and numerical simulation in terms of the major operation and design parameters. Fine-thermocouple, laser-induced fluorescence (LIF) and Raman scattering are used to measure the temperature and major species and OH concentration data at the tube exit. The experimental results show that the tube-exit temperature increases with fuel concentration, velocity, and tube size. For high fuel concentration and velocity cases in the 1000- and 500-mum tubes, an obvious gas-phase reaction behind the exit can be detected by thermocouple and LIF-OH images. Numerical simulation results show that smaller tube sizes and lower velocities would enhance the conversion ratio on the catalytic surface due to the enhanced diffusion of surface species of H-2 and O-2. Based on the current results and analysis, the characteristic operation regions of hydrogen catalytic combustion in microtubes are quantitatively identified in terms of parameters related to heat generation and heat loss characteristics, competition among the timescales, and tube size. Decreasing the tube size will shift the operation region toward the high-concentration and high-velocity portion of the domain with a smaller operation area.