Diffusion coefficients of erucamide (13-cis-docosenamide, H3C(CH2)(11)HC=CH(CH2)(7)CONH2, eru) into isotactic polypropylene (i-PP) were determined under different isothermal conditions, viz., 323, 333, and 353 K in times of 15 days. Some other experiments were performed for several predetermined time intervals (5, 10, and 15 days) at a constant temperature (343 K). The diffusion cell which has been used to establish eru concentration profiles of diffusant within the polymer matrix is similar to that described and used by Roe et al. The concentration profiles of additive in film stacks have been analyzed after soxhlet extraction by capillary gas-liquid chromatography (CGLC). On one hand, these profiles were compared with theoretical curves based on solutions of Fick’s diffusion equation for the best fitting, with the appropriate boundary conditions. However, the measured concentration profiles do not show a good agreement with the Fickian law as will be desirable. Values of the diffusion coefficient D in the range from 10(-9) to 10(-11) cm(2) s(-1) have been obtained. On the other hand, probably due to a dual mechanism and/or some other characteristic effects of the diffusion of erucamide through i-PP it is to be expected that the diffusion process itself may be more complex than that predicted by a simple Fickian model. In this paper we present the diffusion or migration of erucamide through i-PP by means of a semiempirical dual mechanism, in which two simultaneous Fickian processes, perhaps with two types of diffusant molecules, with diffusion coefficients D-1 and D-2 are involved. Some plausible explanations are given at this respect. Similarly, we have estimated by means of the method of Matano an overall diffusion coefficient which is related to the erucamide concentration. Furthermore, this overall diffusion coefficient has also been related to the concentration of erucamide and temperature by using an empirical model. The dependence of diffusion coefficients D, D-1, and D-2 on temperature followed Arrhenius-type relationships over the temperature range investigated. The activation energies were calculated as E(d)(D) = 117, E(d)(D-1) = 100 and E(d)(D-2)= 130 kJ mol(-1), respectively. The values of diffusion coefficients and activation energies are in the range found for some other additives. The solubility of erucamide in i-PP as a function of temperature has been determined directly by extrapolation of diffusion data to penetration depth equal to zero. From the plot of these solubility parameters as a function of temperature, it is very easy understood that erucamide is insoluble (incompatible) within i-PP for temperatures below 313 K. By using Fujita’s equation a good correlation between diffusion coefficient and free volume fraction 1 estimated by means of Williams-Landel-Ferry’ equation has been found.