The fundamental induced infrared absorption of O-2 physisorbed on annealed NaCl films is observed for the first time. Analysis of spectroscopic and thermodynamic data is consistent with molecules adsorption on defect sites and on Na+, Cl-, and the center of lattice square of smooth NaCl(100) faces of the film crystallites. Isotherms of O-2 On three smooth-face surface sites at 58 and 60 K are reported with isosteric heats of adsorption of -9.0+/-1.0, -7.1+/-1.0, and -7.1+/-1.0 kJ/mol, respectively. The infrared bands of molecules adsorbed on NaCl films are close to the gas phase fundamental vibrational frequency.