AbstractThe solubility of vinyl chloride monomer (VCM) in PVC powders has been studied by equilibrium vapor pressure and microbalance gravimetric techniques at temperatures from 30 to 110°C. At temperatures and VCM concentrations above the glass transition, the solubility closely follows the Flory‐Huggins equation with χ = 0.98 and is independent of temperature and of the PVC type, molecular weight, or history. In the glassy state, the VCM solubility is higher than the Flory‐Huggins value and shows pronounced dependence upon time and the PVC history. These results have been interpreted through the dual‐mode sorption concept of Michaels, Vieth, and Barrie: Normal dissolution follows the Flory‐Huggins relation, and the additional glassy‐state solubility represents the contribution of a hole‐filling process. Changes in solubility with time and sample history parallel well‐known volume relaxation processes, indicating that vapor solubility measurements offer a direct and sensitive measure of the free‐volume state of glassy polymers.