We characterize temperature and water vapor mole fraction in the output of a catalytic combustor. Catalytic combustion is used extensively in power production, propulsion, and industrial treatments. Wavelength modulation spectroscopy enables sensitive, non-intrusive, and fast measurements of temperature and water vapor mole fraction in the flow above the combustor. The sensor is first validated under known conditions in a controlled furnace. We then measure the temporal and spatial profiles of temperature and water vapor mole fraction at fuel lean conditions in a model combustor to quantify stability and uniformity, and develop boundary conditions for computational fluid dynamic studies of the flow.