Using different crystallization conditions ZSM-5 zeohte of different crystallite sizes (~1μ and≤0.1μ) was synthesized from the reaction mixture of water glass, aluminum sulphate and ethyl (or n-butyl) amine. It was shown that ZSM-5 crystallites of these two sizes have similar adsorptive capacity, but the smaller crystallites(≤0.1μ) possess higher catalytic activity than the larger ones (~10μ) in the liquid-phase isomerization of gylene.