For design of gas phase polymerization processes for propylene copolymers, it is essential to understand how monomer solubility and diffusivity in the polymer are affected by comonomer content and polymer crystallinity. Measurements of solubilities and diffusion coefficients of ethylene in four propylene-based (ethylene and 1-butene) copolymer (cPP) samples are reported with a gravimetric method at temperatures below and above the melting point temperatures of each cPP. Diffusion coefficients were correlated with free-volume-theory adopting the free volume fraction evaluated from PVT data and the glass transition temperature of polymer. In the polymer rubbery state, solubilities and diffusion coefficients of ethylene could be predicted with the cPP crystallinities and FVT parameters obtained from molten state data. (C) 2019 Elsevier B.V. All rights reserved.