Diffusion and equilibrium coefficients in ternary polymer-solvent-solvent systems were measured using inverse gas chromatography (IGC). The technique provides a new approach to probe diffusion in ternary polymer-solvent-solvent systems. The self-diffusion coefficients of a solvent in a mixture of a polymer and a second solvent were measured. Experiments were performed for the diffusion of toluene and methanol in the poly(vinyl acetate) (PVAC)-toluene-methanol system and for diffusion of toluene and heptane in the polybutadiene (PDB)-toluene-heptane system. Predictions of diffusivities in ternary systems based on the free-volume theory are also critically analyzed and compared with the experimental data.