A study of the nature and accessibility of internal and external protonic and cationic sites has been carried out on protonic and partially Co-exchanged FER, MFI, and MOR through the use of UV-vis and FT-IR spectroscopy. The ion-exchange procedure was exactly the same for the three zeolites. A sample of Co-exchanged silica-alumina has also been investigated for comparison. UV-vis spectra recorded after outgassing at 773 K provide evidence of the presence of low-coordination Co2+ sites in all outgassed Co-containing samples. However, they show that the sites located in the zeolite cavities cannot be distinguished from those located at the open surfaces. FT-IR studies of the adsorption of benzonitrile and o-toluonitrile, used as differently hindered weak basic molecules, allowed us to distinguish sites located at the external surfaces or in different cavities. Benzonitrile enters only the main channels of H-MOR and Co-H-MOR, whereas it is not able to enter the mouths of the side pockets. Benzonitrile also enters the channels of H-MFI and Co-H-MFI. o-Toluonitrile enters the main channels of H-MOR but does not enter the main channels of Co-H-MOR. Similarly, it slowly enters the channels of H-NIFI but does not enter those of Co-H-MFI. So, it provides evidence for the narrowing of the pores of MOR and MFI by Co-exchange. Both nitriles do not enter at all the channels of H-FER and Co-H-FER but provide evidence for the location of Co ions at the outer surface. In all cases terminal silanols and Lewis sites are located at the external surfaces, whereas the strongly acidic bridging OHs are exclusively located in the interior of the cavities. In Co-H zeolites, Co2+ ions are distributed between internal and external surfaces. The extent of exchange of the internal OHs roughly depends directly on the cavity dimensions, increasing in this sense: FER < MFI < MOR and MOR (side pockets) < MOR (main channels). The data also show that, at least for MFI, cation exchange essentially occurs at the mouth of the channels.