The effect of the transport of a series of n-alkane and a series of ester penetrants on the dynamic mechanical response of a polyamide-based polymer matrix has been investigated. This was done by comparing the dynamic mechanical behaviors of the polymer exposed to the penetrants for various times ranging from 0 to 10(5) min. The concept of double reduction in variables (using temperature and penetrant exposure time) was adopted to create master curves of the dynamic loss moduli of the polymer-penetrant systems that were shifted with respect to both of the above-mentioned variables. This procedure involved the introduction of a new shift factor, diffusion-time shift factor, designated as a(Dt). Diffusion-time shift factor plots (log aDt vs. log time) were made for each of the polymer-penetrant systems, and analysis of these curves revealed that there is no change in the polymer relaxation mechanism due to increased exposure time to a given penetrant as well as penetrant size. Differences in chemical nature between the n-alkanes and esters (i.e. due to ester group) were also found not to alter the mechanism of molecular relaxation. However, the chemical nature of the esters indeed influences the rate and extent of polymer relaxation. The dynamic mechanical behaviors of the polymer-penetrant systems from the present study have also been correlated with the basic relationships between the molecular structure (size, shape, and chemical nature) of the penetrants and their observed diffusion properties that were established in the previous publications [Kwan, PhD Dissertation, 1998; Polymer 44 (2003) 3061; Polymer 44 (2003) 3071]. These correlations were used to demonstrate that it is possible to obtain the diffusion parameters (D, D-0, E-d, M-infinity, and concentration profiles) of the polymer-penetrant system, as well as the effect of penetrant transport on the dynamic mechanical response (G", log a(T), and log a(Dt)) of the polymer matrix, based upon knowledge of just the penetrant structure. (C) 2003 Elsevier Ltd. All rights reserved.