The chemisorption of NO on Pt{110}-(1x1) and -(1x2) has been studied using density functional theory slab calculations with the generalized gradient corrections. On both surface phases the monomeric species is the most stable and the short-bridge sites are energetically the most favorable adsorption sites. Monomeric NO is adsorbed upright with its molecular axis normal, bonded to the surface through the N atom. On the (1x2) surface at high coverage a polymeric (NO)(x) chain structure is identified; this may well correspond to the structure experimentally observed at high coverage on the (1x2) surface formed after heating a multilayer to temperatures between 80 K and 200 K, characterized by an NO IR band at 1760 cm(-1).