Several studies of melt rheological properties of polystyrene have been conducted over the past 50 years. Several approaches, including empirical models, have been developed to understand the behavior of materials using simple equations. The existing melt rheology models are best suited for high-molecular-weight polymers whose T-g does not vary. In this work, a semiempirical viscosity equation has been derived, including the effect of T-g dependence on molecular weight, to describe the melt rheology of low-molecular-weight polymers. The equation is derived based on a combination of well-known concepts, such as the effects of free volume and molecular dynamics on polymer rheology. This provides a better understanding of the rheological behavior in the low-molecular-weight regime with respect to temperature and molecular weight. Because of the industrial trend towards lower molecular weight materials for applications such as high solids coatings, this unifying approach, based on the free volume theory with a simple expression, is of extreme practical significance. This equation can predict the zero shear viscosity behavior for different molecular weights, including low-molecular-weight regions, and temperatures. Viscosity calculations using the empirical equation agree with published experimental data. (c) 2006 Wiley Periodicals, Inc.