A semi-analytical solution for the concentration profiles of the main reactants and products in a catalyst particle in the presence of coke formation is developed. The solution is translated into a global deactivation function over the particle, which is related to the effectiveness factor appearing in the fluid-phase model equations. The paper considers both parallel and consecutive coking and deactivation functions for the main and coking reactions, which are linearly and exponentially dependent on the coke content. The solution is compared with results obtained by numerical integration. The agreement is excellent, so that the problem of diffusion, reaction and deactivation by coke formation can now be dealt with through an approach requiring a minimal computational effort only.