Immobility of heavy oil due to its high viscosity leads to a low recovery in such reservoirs. Carbon dioxide injection can be used to enhance oil recovery by reducing its viscosity as the gas mixes and diffuses into the heavy oil. Better understanding of diffusion coefficient of CO2, Dco2, in porous media, which is a significant issue in recovery factor of oil fields is the main objective of my work. In this thesis, I have analysed the Dco2 in porous media under initial conditions via physical experiments. CO2 gas was injected into a container/core holder containing water/oil saturated Berea sandstones at temperature of 40, 80°C and pressure of 100 bar. As the CO2 molecules start to diffuse in the porous media, the pressure in the system changes. The change of pressure over the time depends on the rate of diffusion that means on the diffusion coefficient and therefore the subsequent pressure decline was monitored to be used in a mathematical form to interpret the diffusion coefficient. The mathematical model was developed using fick’s law combined with gas law and at the end the diffusion coefficient was calculated using pressure profiles coupled with the defined mathematical model. Preliminary experiments with water/brine were run at pressure of 50, 100, 200 bar and temperature of 40, 60, 80°C to check the experimental set up and mathematical model. The diffusion coefficients calculated by these experiments were compared with the reported values in the literature. Cussler, 1976 and Reid et al.,1977 have found Dco2 in water at 25°C. Comparison of their value, 1.92E-9 m2/s with my value, 4.86E-9 m2/s at Temperature of 40°C agrees well with stokes Einstein equation that says the diffusion coefficient increases with the temperature. Also, the results for pressure of 50,100, 200 bar respectively with values of 4.86E-9, 9.6E-9, 8.06E-8 m2/s show that the diffusion coefficient is increasing with pressure, i.e., the initial concentration of CO2 in the system and indicate the dependency of diffusion coefficients on concentration. Experiments for porous media all have been done at 100 bar and compared with the experiments for oil. For instance the results of Schoenkirchen oil has the value of 1.5E-8 m2/s Whereas the same experiment for saturated Berea sandstone at the same condition has the value of 8.03E-10 m2/s. That shows the Dco2 in porous media has lower value when compared to oil.