The void structure of zeolite MCM-22 is discussed on the bases of catalytic reaction tests. The isomerization/disproportionation of m-xylene on the acidic zeolite, and the hydroisomerization of n-decane on bifunctional Pt/zeolite catalysts have been used as model reactions. The conversion of n-decane has been carried out under more realistic hydroisomerization conditions, i.e., higher pressure and lower H-2/n-decane ratio, as compared to the original work. Under these conditions it has been shown that the n-decane test is also a useful reaction to differentiate between different zeolite structures. From both reaction models it is concluded that zeolite MCM-22 shows features of both 10 MR and 12 MR zeolites depending on the different selectivity criteria used. This could be explained, in principle, by assuming the presence of both types of channels in the structure of MCM-22 which are probably leaving free spaces of a higher diameter than the channels.