An extensive grand canonical Monte Carlo (GCMC) simulation combined with quantum mechanics calculations has been conducted to investigate the dependency of the adsorption isotherms, local density profiles, and orientational behaviors of HCFC-22 confined in pillared clays on pore width, porosity, pressure, and the density of the activated sites at 300 K. The calculated results suggest that there are optimal pore width and pressure for adsorption, and the effects of porosity and the activated sites on adsorption are significant at low pressure and become weaker with increasing pressure. A combination of the GCMC simulation and the quantum mechanics calculation results shows that hydrogen atom is the adsorption site of an HCFC-22 molecule adsorbed on the pillared clay walls and most of the molecules in the contact layers orient perpendicularly to the walls in the direction of C --> H when physical adsorption is predominant, and the orientation changes with increasing pressure. (C) 2004 American Institute of Chemical Engineers.