Theoretical and experimental studies of the change of diffusion coefficient in porous media are reported. Mathematical equations were derived for calculating the diffusion coefficient in oil and gas phases. Experiments were conducted by measuring the pressure decline in both a PVT-cell and a sample of actual core. The medium porosity was found to have a significant effect on the system pressure decline. The diffusion coefficient in the PVT-cell was two orders of magnitude greater than that in the core. The diffusion coefficient was studied as a function of rock permeability and porosity. It was found that the porosity was the main factor affecting the diffusion coefficient whereas the permeability could be neglected.