Vapor-liquid equilibria and mutual diffusion coefficients have been measured for the polyisobutylene + toluene system from 5 ° C to 65 ° C by gravimetry. Empirical relations have been determined for the Flory interaction parameter and for mutual diffusion coefficients. They show very good agreement with experimental data and can be accurately used for simulation or other purposes as long as they are restricted to the concentration and temperature ranges covered by our study.