New graphical techniques are presented for estimating the diffusivity coefficient (or mass diffusivity) Of CO2, CH4, and N-2 in highly viscous bitumens from pressure-decay data. These methods are based on modeling the rate of change in system pressure as gas diffuses into the bitumen using the diffusion equation, coupled with a mass balance for the gas phase. Analytical solutions of the resulting set of equations, with appropriate initial and boundary conditions, are obtained by Laplace transformation. An inverse solution technique is used for developing two graphical methods to estimate the diffusivity coefficient from pressure-decay data reported in the literature. The estimated diffusivity coefficients for gas-bitumen pairs at 75-90 degrees C vary from 2.5 x 10(-10) m(2)/s to 7.8 x 10(-10) m(2)/s, and these are in good agreement with literature values. The novelty of the proposed methodology is in its simplicity and its ability to isolate portions of the pressure-decay data that are affected by experimental fluctuations. This enables the consideration of only that portion of the data that is consistent with the analytical solution.