The influence of surface structure on NO chemisorption and dissociation on Pt{100}-(1 x 1), Pt{211}, and Pt{410} has been studied using density functional theory slab calculations with the generalized gradient corrections. The presence of steps on Pt{211} strengthens the NO-surface chemisorption bond, but the barrier for NO dissociation remains high. On the other hand, the steps on Pt{410} help to stabilize the N and O adatoms that form upon dissociation and the transition state. The calculated barrier of 80.2 kJ/mol on Pt{410} is in good agreement with experiment. These results show that both the presence of steps and the nature of the steps are important to activate NO. An ensemble of square-arranged Pt atoms has been identified as an important feature in activating the N-O bond.