This work provides the experimentally determined composition-dependent diffusivity of environmentally benign plasticizer such as carbon dioxide in polypropylene. For this purpose, a pressure decay system is employed to determine composition-dependent diffusivity of carbon dioxide in polypropylene at isothermal conditions. On the basis of a detailed mass transfer model, variational calculus is used to establish the conditions necessary to yield the composition-dependent diffusivity that enables the model-predicted mass of absorbed gas in polymer to match with the experimental counterpart. The experimental solubility and composition-dependent diffusivity of carbon dioxide in polypropylene are obtained at (170, 180, and 190) degrees C for pressures up to 7.32 MPa.