The pressure-decay technique is used to determine the diffusivity and solubility of methane and carbon dioxide in pure hydrocarbons and bitumen at temperatures of 0, 15, 20, and 25 degrees C and pressure of 3.5 MPa. An analytical graphical technique is implemented to extract the related mass-transfer parameters, i.e., diffusion coefficient and Henry's constant, from the pressure-decay data. The results reveal that the diffusion coefficient of methane and carbon dioxide in both pure hydrocarbons and bitumen decreases as the temperature decreases. On the basis of the estimated Henry's constants, the effect of the temperature on the solubility of the gaseous solvents in bitumen and pure hydrocarbons is also determined. As expected, the solubility of both gases in the studied pure hydrocarbons increases as the temperature decreases. In addition to confirmation of the known trends, this study provides new experimental data for the low-temperature gas hydrocarbon diffusion process.