Alkaline post-treatment (0.2 M NaOH at 353 K) of ZSM-5 leads to substantial formation of mesoporosity. The created mesopores do not have a uniform size of 4 nm,(1) but show a broad distribution around 10 nm. The sharp peak at 4 nm in the pore size distribution derived from the N-2 desorption isotherm should be attributed to the Tensile Strength Effect of the adsorbate instead.