The alkylation of solid 4-tertiobutylphenol by gaseous isobutene in the presence of a catalyst has been studied according to different parameters such as, temperature, initial pressure of isobutene, proportion of catalyst and the granulometry of the catalyst and the solid reactant. Among the predefined kinetic models, the unreacted core model for Spherical particles of unchanging size is the best adapted to the experimental results. Based on this model, a mechanism is proposed involving the intervention of a carbocation.