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Understanding the sorption equilibrium of penetrants into a polymer is key to mastering the kinetics and properties of the final product. These systems are non‐ideal meaning the presence of multiple components in the continuous phase changes the solubility of individual species with respect to pure component solubilities. For this reason, measurements of the individual and total solubilities of multiple penetrants in isotactic polypropylene (iPP) and random copolymers (RCP) were performed using a pressure decay method at temperatures of 75–85°C, and total pressures of up to 29 barG. The results showed that the pure component solubilities of propylene and propane in amorphous iPP were similar, but 4 to 5 times higher than that of ethylene in the same polymer. However, in the ternary system of propylene/propane/iPP, it is seen that propylene acts as an anti‐solvent to propane, and that propane acts as an anti‐solvent to propylene, resulting in lower solubility of both penetrants than in the respective binary cases. The co‐solubility effect of propylene on ethylene and the anti‐solvent effect of ethylene on propylene were observed for the system of propylene/ethylene/RCP. [ABSTRACT FROM AUTHOR]