The definition of a chemical potential in nonequilibrium situations raises a problem about the quantities to be held fixed during the differentiation of the generalized Gibbs free energy. In this paper we compare two different definitions of the nonequilibrium chemical potential of polymers under shear flow, starting from the experimental data by Rangel-Nafaile et al. concerning phase-separation experiments, under shear. The definition at fixed shear viscous pressure turns out to be more suitable than the definition at fixed conformation proposed previously in the literature by some authors. © 1991, American Chemical Society. All rights reserved.