The effects of complex-oxide crystalline phase on catalytic activity and the pathways of NO oxidation are investigated by comparing SmMn2O5 mullite and SmMnO3 perovskite crystals. Synthesized under the same conditions, SmMn2O5 shows activity at lower temperature (200 degrees C) with a higher maximum conversion efficiency compared to SmMnO3 (52% vs. 36%), inspite of similar active site density. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) results show nitrates as the primary surface reaction species. FTIR, XPS, and temperature programmed desorption (TPD) measurements all indicate that nitrate species are less stable on SmMn2O5 (< 250 degrees C) compared to SmMnO3 (> 300 degrees C). Results from density functional theory calculations show that the barrier for nitrate dissociation is much higher on the Sm-terminated (001) surface of SmMnO3. Combined experimental and theoretical findings suggest that the superior catalytic performance of SmMn2O5 mullite arises from the ability to regenerate its active sites with nitrate dissociation at lower temperatures for subsequent NO oxidation reactions.