Although the relationship between the relaxation timescales of thermodynamic; mechanical, viscoelastic, and dielectric properties in amorphous materials has been studied extensively, no general consensus has been reached. In this work, we examine the relationship between the timescales of volume and enthalpy relaxation for polystyrene using the cooling rate dependence of the glass transition temperature (T-g) obtained from capillary dilatometry and differential scanning calorimetry (DSC). Our analysis suggests that both volume and enthalpy exhibit similar relaxation timescales at temperatures above and below T, The divergence of the times required to reach equilibrium noted in the literature at temperatures several degrees below the nominal Tg is attributed to the effects of nonlinearity. The relationship between nonlinearity and dynamic heterogeneity is discussed. (c) 2007 Published by Elsevier Ltd.