The mutual diffusivity of solvent and bitumen is an important parameter in the design of solvent-based bitumen recovery processes. It is determined from measured concentration profiles in bitumen. These measurements are difficult because bitumen is opaque and current approaches such as X-ray or magnetic resonance tomography are expensive. A new apparatus was designed and commissioned as a lower cost alternative. It measures the mass transfer in liquid solvent/heavy oil systems based on the density profiles established over time in a column of solvent over bitumen. A one-dimensional numerical model based on molecular diffusion was developed to determine the mutual diffusivity from the concentration profiles. The model accounted for the dependence of diffusivity on viscosity through a correlation based on the infinite dilution diffusivities of the solvent and the oil. Mutual diffusivities were determined for Athabasca bitumen with toluene and for maltenes from the same oil with toluene, n-heptane, and n-pentane at ambient conditions and diffusion times from 3 to 15 days. The measured diffusivities were found to increase monotonically with decreasing viscosity (and solvent content) of the mixture but remained below the self-diffusion coefficient of the solvent.