The gas-phase ignition of fuel-lean hydrogen/air mixtures over platinum was investigated experimentally and numerically in laminar channel-flow configurations. Experiments were performed at atmospheric pressure in an optically accessible catalytic channel combustor established by two Pt-coated parallel plates, 300 mm. long (streamwise direction) and placed 7 mm apart (transverse direction). Planar laser induced fluorescence (PLIF) of the OH radical along the streamwise plane of symmetry was used to monitor the onset of homogeneous ignition, one-dimensional Raman measurements (across the 7-mm. transverse direction) provided the boundary layer profiles of the major species and temperature, and thermocouples embedded beneath the catalyst yielded the surface temperature distribution. Computations were carried out using a two-dimensional elliptic fluid mechanical model that included multicomponent transport and elementary homogeneous (gas-phase) and heterogeneous (catalytic) chemical reaction schemes. Four homogeneous and three heterogeneous reaction schemes were tested in the model against measured homogeneous ignition characteristics. The differences between measured and predicted homogeneous ignition distances could be substantial (ranging from 8% to 66%, depending on the particular hetero/homogeneous schemes) and were ascribed primarily to the homogeneous reaction pathway. Sensitivity analysis indicated that the discrepancies induced by the gas-phase schemes originated either from the presence of heterogeneously-produced water due to its effectiveness as collision partner in the chain terminating reaction H + O-2 + M = HO2 + M, or from an overall overprediction of the radical pool in the preignition zone. The heterogeneous schemes had significant differences in their surface coverage and radical fluxes, but these variations had practically no impact on homogeneous ignition. Sensitivity and reaction flux analyses have shown that this was attributed to the ability of all heterogeneous schemes to capture the measured mass-transport-limited fuel conversion and to the relative insensitivity of homogeneous ignition on the magnitude of the heterogeneous radical fluxes, provided that all radical adsorption reactions (OH, H, and O) were included in the heterogeneous schemes.