beta-Glucan isolated from baker's yeasts (Saccharomyces cerevisiae) and chitosan films were subjected to stress relaxation tests performed at different prestress levels with the aim to study the activation volume during relaxation and to establish corresponding relaxation models. Apparent activation volume (AAV) was numerically calculated from the slope of the relaxation curve and analyzed in dependence on the stress. The tested films conformed to the standard Cottrell-Stokes' law as a reciprocal proportion between initial AAV and prestress. The validity of the Cottrell-Stokes' law could be extended to the main part of relaxation in -glucan films stored in standard conditions and associated power relaxation models were applied on experimental data. This behavior was not observed in the case of chitosan films and -glucan films which were long stored under dry conditions. The indicated differences in the inner structure of -glucan films were supported by the X-ray diffraction analysis. In chitosan films, the role of prestress location with regard to the yield point was more considerable than in -glucan films. Chitosan films tested beyond the yield conformed to the logarithmic model in the first part of relaxation. POLYM. ENG. SCI., 55:624-633, 2015. (c) 2014 Society of Plastics Engineers