Low-temperature (Ο½250 ΠC) combustion over Pd catalysts of very lean H 2 /O 2 mixtures diluted in N 2 has been studied experimentally using a microreactor with transient exhaust monitoring using mass spectroscopy. Experimental results using a c-Al 2 O 3 washcoat-supported PdO x catalyst reveal the importance of transient measurements for elucidating features of the catalytic combustion mechanism and, in particular, the effects of H 2 O adsorption/desorption and of the Pd oxidation state. For the cases studied, experiments indicate that H 2 conversion depends on equivalence ratio () only at low temperatures (T in Ο½ 125 ΠC) where conversion remains Ο½40%. For T in ΟΎ 125 ΠC, mass transfer limitations become more significant, and thus, as T in rises, conversion becomes relatively independent of . Addition of H 2 O vapor to the inlet flow causes a reduction in conversion for lower T in , but for T in ΟΎ 125 ΠC, it only delays catalyst light-off and the rapid transition to high steady-state conversion. A final set of experiments indicated very high (and apparently steady) H 2 conversion at T in as low as 50 ΠC by starting with a prereduced catalyst. Efforts to understand the experimental results with a transient one-dimensional reactor model using detailed surface chemistry indicates the importance of the relative adsorption rates of H 2 and O 2 as well as the H 2 O adsorption/desorption. The model captures the trends for conversion with respect to temperature but fails to predict well the influence of inlet H 2 O concentrations. Implications on further mechanism development of the discrepancies between the model predictions and experiments results are discussed. Nonetheless, these low-temperature H 2 combustion studies provide a starting point to further Pd surface chemistry for combustion of other fuels for a wide range of applications.