Infrared reflection-absorption spectroscopy and density functional theory, within the generalized gradient approximation, were used to investigate both experimentally and theoretically N-2 chemisorption on stepped and smooth Pt surfaces. N-2 chemisorption was observed to occur only on the edge atoms of step defect sites in atop configuration by both methods. The calculated vibrational frequency of N-2 chemisorbed on Pt(112) step sites (2244 cm(-1)) is in good agreement with the frequency observed experimentally (2231-2234 cm(-1)) at saturation coverage on Pt(335) and Pt(779). The predicted small N-2 binding energy confirmed its weak chemisorption on Pt surfaces claimed in previous studies. The calculations indicate that N-2 decreases and CO increases the work function of the Pt(112) surface. N-2 could be coadsorbed with CO below saturation coverage of the steps with CO and there is a charge transfer between the two adspecies through the substrate.