The deactivation kinetics of toluene alkylation with methanol over a Mg-modified ZSM-5 catalyst has been studied. A kinetic model taking into account both the deactivation of the main and the secondary reactions and the influence of the intracrystalline diffusion has been developed. The best fit of the experimental data has been obtained assuming that gaseous hydrocarbons, formed mainly by ethylene, are the coke precursors. The secondary reactions of p-xylene dealkylation, toluene disproportionation, and external p-xylene isomerization deactivate faster than the main reaction, probably due to differences in the strength of the acid sites over which they take place. The paraselectivity corresponding to the primary product decreases with the time on stream as a consequence of the pore blockage by coke, which attenuates the diffusional control of the internal xylene isomerization. Methanol dehydration is the reaction least affected by coke.