The interaction of different probes with two H-MOR samples has been studied by IR. In the case of the sample with Si/Al = 10 acetonitrile perturbs all the hydroxy groups while 2,2-dimethylpropionitrile (pivalonitrile) perturbs only very few. Pyridine also perturbs all the hydroxy groups but only some of them protonate the pyridine, the others only H-bond to it. n-Hexane and 2,2-dimethylbutane give the same result, perturbing only some of the bridging hydroxy groups. The results are interpreted by assuming that no OHs are located in the 8-ring channels, and that the hydrocarbons cannot interact (due to the steric hindrance of the methyl group) with the OHs located in the side pockets. On the contrary, the at molecule pyridine can enter slightly into the side pockets and H-bond with the OHs there. Pivalonitrile interacts only with the OHs which are well exposed in the main channels. It is concluded that the active sites for alkane isomerization are likely exclusively those that are well exposed in the main channels of H-MOR and that Al substitution in the T3 sites probably does not occur. The sample with Si/Al 45, taken as an example of a dealuminated sample, presents many less bridging OHs which are entirely available for interaction with even pivalonitrile