Diffusion mechanism of N₂, CH₄ and CO₂ in pelletized zeolite Ca-X was studied theoretically and experimentally in the range of 0-1.0 atm and 273-293 K. Using the isothermal and nonisothermal adsorption rate models, diffusion coefficients were obtained from the experimental uptake curves through a gravimetric method. The effect of heat of adsorption on the adsorption rate was also investigated using these mathematical models. The diffusion showed the tendency of an isothermal behavior in the range of low pressures, but showed a nonisothermal behavior with an increase of pressure due to the effect of heat of adsorption. The adsorption rates of N₂, CH₄ and CO₂ in pelletized zeolite Ca-X were controlled by the macropore diffusion. In the case of N₂ and CH₄ at low temperature and pressure, Knudsen diffusion was dominant in the effective macropore diffusion, while the controlling mechanism of the effective macropore diffusion was moved to the transition region of Knudsen diffusion and a molecular diffusion with an increase of temperature and pressure. In the case of CO₂, however, the diffusion rate control in the effective macropore diffusion showed a transition from Knudsen diffusion to a molecular diffusion with an increase of pressure. Furthermore, the effective diffusion coefficients in the macropore showed dependences on both pressure and temperature.