The adsorption and subsequent dissociation of O-2 on Pt(111) was studied by variable temperature scanning tunneling microscopy in the temperature range of 40 to 215 K. Tight clustering of bridge site molecules is observed on terraces between 40 and 70 K, indicating a highly mobile precursor to chemisorption. Coexistence of bridge and fee hollow site molecules in fractal-shaped islands is observed after dosing between 70 and 95 K. Dissociation of these species was induced by uv radiation, inelastic tunneling electrons, and heating. In all three cases, two O atoms are found within two lattice constants of the original molecule and one to three lattice constants apart. (C) 1997 American Institute of Physics.