The rate-determining factors for isothermal structural relaxation in non-crystalline materials are discussed. The normalized volume and enthalpy relaxation rate RF is defined and analyzed for the Tool-Narayanaswamy-Moynihan phenomenological model. It is shown that the temperature dependent RF is controlled mainly by the parameter (1-x)theta, where x is the nonlinearity parameter and theta = Delta h*/RTg2. Materials with higher values of parameter (l-x)theta (e.g, vinylic polymers) exhibit low relaxation rate. In contrast, high relaxation rate is expected for materials with low values of (l-x)theta (inorganic glasses). The applicability of this approach for the analysis of isothermal volume and enthalpy relaxation data is shown for arsenic sulfide glass, polystyrene, poly(vinyl acetate), poly(methyl methacrylate) and poly(vinyl chloride). The normalized enthalpy and volume relaxation rates for these materials agree well within the limits of experimental errors.