Primary causes of deactivation of hydrodemetallation catalysts are partial poisoning of the interior surface and pore plugging by deposition of metals. Local changes in effective diffusivity and reaction rate constant should be taken into account, to describe the drop in activity by intraparticle diffusion and reaction. Local reaction rate constant varies with both changes in surface area and surface activity, along the position within the catalyst particle. Attempts are made to describe mathematically the deactivation of catalyst, resulting dimensionless equations for those changes. Derived from the probability of void and deposition, reduction of local effective diffusivity and shrinkage of local surface area are formulated as functions of deposits. Included parameter is turned out to be the ratio of size of organometallic compounds to the pore diameter. The change in surface activity is also given as a function of deposits, from the kinetics of surface fouling and deposition. The treatment is for a single, isothermal catalyst particle.