The Haasen plot (inverse activation area 1/Delta alpha vs. offset flow stress sigma-sigma(s)) for solute-strengthened alloys is usually assumed additive, 1/Delta alpha = 1/Delta alpha(s) + 1/Delta alpha(f), with 1/Delta alpha(f) similar to beta(sigma-sigma(s)) due to forest interactions. Experiments often show a slope textlessbeta. Here, a model for the dislocation activation enthalpy is proposed that predicts a slope 1/(Delta alpha(s)sigma(s)) determined only by solute parameters Delta alpha(s) and sigma(s) and not directly connected to forest hardening. This parameter-free prediction agrees well with a wide range of experiments on Al-X alloys at T = 78 K. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.