The catalytic performance of MCM-22 modified with Cs to different extents by Solid-State Ion Exchange has been investigated in toluene disproportionation. It was suggested that protons on the external surface were preferentially replaced by Cs cations. It was observed that the Cs modifications did not provide improved p-xylene selectivity at conversions close to that of the H-form. The remaining unaffected proton sites in the 10 MR channels are considered as responsible for secondary xylene isomerization proceeding upon toluene disproportionation.