The interaction of O2 with Pt(100)–hex and (1×1) surfaces at 123 K was studied by thermal desorption spectroscopy, x-ray photoemission, and work function techniques. On the hex surface, oxygen is adsorbed entirely as the molecular state with a sticking coefficient of 0.13 and exhibits an activation energy for desorption ∼37 kJ mol−1. The saturation coverage is 0.24×1015 O2 molecules cm−2 and desorption does not lead to any dissociation. On the (1×1) surface, oxygen dissociates, even at 123 K. The sticking coefficient is 0.4, and the saturation coverage is 0.48×1015 O atoms cm−2. On warming to 240 K, a (2×1) LEED pattern is observed that is believed to originate from an oxygen overlayer on an unreconstructed substrate. At ∼360 K, the surface transforms irreversibly to the oxygen stabilized (3×1) reconstructed phase.