Experimental determination of solubility and diffusion coefficients of CO(2)and N-2 in poly(dimethyl, methylphenyl siloxane) (PMPS) for the pressure range 0-150 kPa and the temperature range of -10 degrees C to 45 degrees C is discussed. The absorption isotherms were measured by pressure decay equilibrium. Diffusion coefficients were determined by modeling the transient absorption data by Fickian diffusion. The value of solubility coefficient for CO2 at 35 degrees C was 1.21 +/- 0.02 cm(3) CO2 (STP)/(cm(3) polymer.atm) and it increased to 2.07 +/- 0.01 cm(3) CO2 (STP)/(cm(3) polymer.atm) at 1 degrees C. The absorption isotherms of CO2 obtained were linear for the pressure range 0-200 kPa and the enthalpy of sorption value was -10.82 kJ/mole for CO2. However, the diffusion coefficients Of CO2 changed with feed pressure. The diffusion coefficient of CO2 in PMPS at 22 degrees C and 128.6 kPa equilibrium pressure was 7.22 x 10(-6) cm(2)/s and thus the permeability value at these conditions was 1392 Barrers. The diffusion coefficients of CO2 at 0 degrees C and equilibrium pressures of 125.8 kPa and 10.5 kPa were 5.72 x 10(-6) cm(2)/s and 4.31 x 10(-6) cm(2)/s, respectively. Consequently, the CO2 permeability in PMPS decreases by about 20% when the pressure is reduced from 125.8 to 10.5 kPa at 0 degrees C. (C) 2007 American Institute of Chemical Engineers.