The dependence of the reaction rates and selectivities in toluene methylation over zeolite HZSM5 upon the concentration and type of adsorbed surface species are reported. Rather low selectivity to p-xylene was found at 473 K, while high selectivity was observed at 573 K. Under reaction conditions which lead to low selectivity to p-xylene (e.g., at 473 K) the reaction can be described best as a network of parallel alkylation reactions. Under reaction conditions which lead to high selectivity top-xylene the steady state concentration of the builder isomers in the adsorbed phase was so high that the secondary reaction, the isomerization of the formed xylenes, in the HZSM5 pores was faster than the rate of alkylation. Thus, we conclude that for shape selective methylation of toluene to p-xylene the rate of isomerization has to exceed the rate of alkylation. High concentrations of adsorbed molecules or coke precursors in the zeolite which certainly increase the spatial constraints of the reacting molecules do not suffice to achieve high p-selectivity. This indicates that transition state selectivity does not play a major role for the product selectivities in the methylation reactions studied.