While chemical surface modification is commonly employed to improve the performance of solid catalysts, this approach has been largely neglected in optimizing photoadsorbents. Here we show that powdered Fe2O3 treated with aqueous NH4Cl and subsequently calcined near 300 degrees C exhibits appreciable activity for the photoadsorption of NO, whereas the pure iron oxide does not. We employ a uniquely configured reactor to develop well-characterized photoadsorption kinetics without shadowing or diffusion effects. Kinetics obey simple Langmuir-type expressions for nondissociative adsorption. However, the adsorption process requires the simultaneous presence of adsorbed chlorine and H2O, and X-ray photoelectron spectroscopy shows that both the +2 and +3 oxidation states of iron play a role. This complexity mirrors corresponding complexity in the bonding of NO to Fe cations in the analogous aqueous-phase coordination chemistry. Interestingly, the photoadsorbent is not poisoned by exposure to SO2 or CO2.