The distribution of reaction products in properly chosen simple test reactions, exploiting the acidic properties of the catalyst, allows to define the value of some characteristic parameters, such as the constraint index (CI), the modified constraint index (CI*) and the spaciousness index (SI), tightly correlated to the structure of zeolite-type microporous catalysts. This permits to clearly distinguish the behaviour of small-, medium- and large-pore microporous materials, the pore diameter of which can be easily ranked on the basis of a very few or even of only one test reaction.