Combustion-based processing of polymer ﬁlms and other materials often requires temporal and spatial uniformity of the combustor. We characterize the temperature and H2O mole fraction of the buoyant jet above a rectangular iron-chromium catalytic combustor using diode-laser wavelength modulation spectroscopy (WMS). The sensor is ﬁrst validated in a tube furnace under known conditions. It is then used to characterize the temporal and spatial variation of the ﬂow exiting the combustor. The H2 O mole fraction measurements incorporate a new pathlength correction approach that uses computational ﬂuid dynamics to account for the changes in jet width. We observe 3% temporal variation of temperature over 60 s and 7% spatial variation along the length of the burner. Vertical proﬁles indicate that additional combustion is likely occurring above the catalytic surface under certain operating conditions. This type of characterization can be used for optimizing the combustor for a variety of catalytic combustion applications.