The liquid-phase mutual diffusion coefficients for Athabasca bitumen (1) + pentane (2), determined by a free diffusion method, and subsequent analyzing the composition profiles established by X-ray transmission tomography are reported at 295 K. Particular care was taken in the analysis of composition profiles. A method that accounts explicitly for the variation of mutual diffusion coefficient and density with liquid composition was employed in order to obtain time-independent diffusion coefficients consistent with relevant theories and exogenous data sets for mutual diffusivities in liquids.