A simple experiment of transient, one-dimensional diffusion of pure butane vapor into bitumen was designed that permitted direct measurement of bitumen swelling and butane uptake with time. The experimental results showed significant swelling as a result of butane uptake by the viscous liquid. A mathematical model that rigorously accounts for the dependence of both density and mutual diffusion coefficient on liquid composition was developed and used to interpret the experimental data. Given the dependence of mixture density on composition, the concentration dependence of the diffusion coefficient of butane in bitumen was determined using least-squares nonlinear regression. It was found that the dependence of diffusion coefficient on concentration cannot be ascertained from butane uptake or bitumen swelling data alone. In isolation, either set of experimental data (bitumen swelling or butane uptake) can be fitted equally well with rather different functions describing the dependence of diffusion coefficient on composition. Validation against experimental data not used to fit model parameters, however, suffices for discriminating between candidate concentration-dependent diffusivities. These findings improve our ability to accurately measure the diffusivity of light n-alkanes in bitumen, information that is relevant to the understanding of in situ heavy oil recovery using hydrocarbon solvents.