For the evaluation of flow stress of magnesium-based alloys AZ31 and AZ91 in hot working processes, a formula is derived by analysing the stress data measured at various temperatures and strain rates. The formula is expressed in a simple form by means of the temperature-compensated strain rate, i.e. the Zener-Hollomon parameter, Z. It is demonstrated that a linear equation of the logarithmic Z fits the flow stress of the alloys at elevated temperatures. (C) 1998 Elsevier Science S.A. All rights reserved.