The colloidal gas-liquid phase behaviour has been studied for aqueous mixtures of well-defined spherical particles with short-ranged repulsions mixed with relatively monodisperse poly(ethylene oxide) polymers. We show that our set of experimental phase diagrams are in quantitative agreement with theoretical predictions using generalized free volume theory [G. J. Fleer and R. Tuinier, Phys. Rev. E, 2007, 76, 041802]. The determination of the equilibrium composition of coexisting phases reveals qualitative deviations between the two, for which we propose a tentative explanation.