This study determines the diffusivity of nitrogen as a function of its concentration in polypropylene at 170, 180, and 190 degrees C in the range, 0.99-6.75 MPa. Isothermal pressure decay experiments are utilized to obtain the uptake of nitrogen by polypropylene with time. These data when used with a detailed mass transfer model, and the principles of variational calculus allow the determination of the nitrogen diffusivity. It is found to be a peak function with values in the range of 3.88 x 10(-9) to 12.94 x 10(-9) m(2) s(-1). Suitable correlations are found to represent the diffusivity in terms of pressure and nitrogen concentration at the three temperatures. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2828-2834, 2011