The dissociative chemisorption of oxygen on Pt{100}-(1X1) has been investigated using supersonic molecular beams with incident translational energies between 0.04 and 0.90 eV over the surface temperature range 300 to 380 K. The adsorption process on the Pt{100}-(1X1) surface is found to differ from that on the reconstructed Pt{100}-hex-R surface in both magnitude and mechanism. The initial dissociative sticking probability is >0.2 on the (1X1) surface for all beam energies and surface temperatures investigated, whereas on the reconstructed Pt{100}-hex-R surface, under corresponding conditions, the initial sticking probability never exceeds 0.003. The initial sticking probability On the (1X1) surface at first decreases with increasing incident energy and then increases as the incident energy exceeds 0.1 eV, in a manner typical of an adsorption process that is precursor mediated at low incident energies and direct at higher incident energies. Precursor-mediated adsorption at low energies is also indicated by scattering measurements and by the angular variation of the initial sticking probability. At high incident energies the angular variation of the initial sticking probability indicates that adsorption is an activated process. Sticking probability measurements as a function of coverage indicate no extrinsic precursor involvement over the energy range studied; repulsive lateral interactions exist between adsorbed oxygen atoms. Thermal energy atom scattering measurements show an absence of island formation upon adsorption, in contrast to adsorption on the hex-R face.