The thermal and photochemical behavior of O-2 chemisorbed on a stepped Pt(335) surface have been compared using temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Molecular O-2 adsorbs preferentially at 85 K on step sites, followed by adsorption on terrace sites. On heating below 250 K, depopulation of terrace sites occurs first followed by depopulation of step sites. Both desorption and dissociation of O-2(a) occur below 250 K. Photolysis of O-2 occurs preferentially on step sites, with a cross section approximate to 3 x 10(-21) cm(2). The chemisorbed O(a) produced by either thermal dissociation or photodissociation of O-2(a) is identical, exhibiting a Pt-O stretching mode at 480 cm(-1) and a frustrated translation mode at 330-350 cm(-1). Two regimes of photodepletion are observed, dominated by photolysis of step bound-O-2 (regime I) and of terrace bound-O-2 (regime II). Regime II is characterized by the presence of interactions of photoproduced electronegative O(a) with remaining O-2(a), causing an increase of the O-O mode frequency due to electron withdrawal from the O-2(a)pi* orbital.