Solubilities of benzene, 2-butanone, and chloroform in polybutadiene (PBD) and three PBD and polystyrene (PS) copolymers at 298.15 K are reported in this paper. Mass fraction data are collected using a quartz-crystal microbalance (QCM) and related to activity using the Flory-Huggins model; the weight-based activity coefficients at infinite dilution are then obtained and related to copolymer composition. The complexities of collecting data using a QCM for rubbery and glassy polymers are also discussed.