Polypropylene copolymer is commonly used in our life and solubility and diffusion coefficient of monomer in polypropylene copolymer are important parameters to optimize the gas phase polymerization process. The objective of this work was to measure solubilities and diffusion coefficients of ethylene in four molten polypropylene based (ethylene and 1-butene) copolymer (cPP) samples at 140 °C and 180 °C and up to 2.5 MPa and to correlate solubilities of ethylene in cPPs with the Sanchez-Lacombe equation of state and diffusion coefficients of ethylene in cPPs with the free-volume-theory reported by Kulkarni and Stern. For the solubility, the Henry's constants at each temperature were to within 1.84% of deviation for all cPPs. The experimental solubilities were reproduced by the Sanchez-Lacombe equation of state to within 1.72% of the average deviation. For the diffusivity, the diffusion coefficients of ethylene in cPPs increased with increasing 1-butene content in cPPs. The difference of the diffusion coefficients can be explained by the difference of free volume fraction estimated from Pressure-Volume-Temperature data and glass transition temperature of the cPPs. In addition, the Kulkarni's free-volume-theory reproduced the experimental diffusion coefficients to within 4.96% of the average deviation.