A gold nanoparticle embedding technique is used to determine how vacuum and pressured carbon dioxide (CO2) affect polystyrene (PS) thin film properties. The pressured CO2 greatly increased the gold nanoparticle embedding depth, possibly due to a low cohesive energy density near the film surface. For the monodisperse PS used in this study (M-n = 214,000), two spin-coated thin films with intimate contact can be bonded below the bulk glass transition temperature (T-g) under CO2 pressure when the embedded depth is larger than half of the gyration radius of PS molecules. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1535-1542, 2009