AbstractThe zeolites form a numerous group of structurally diverse, porous, crystalline aluminosilicates. The naturally‐occurring zeolites arise by hydrothermal action of solutions in basaltic rocks and also as large sedimentary deposits. The naturally‐occurring species have been augmented by a number of synthetic zeolites without any known natural counterparts; and a range of unknown zeolite structures can also be predicted. The interesting molecular sieves are found both among the naturally‐occurring and the synthetic phases. An account has been given illustrating typical ways of representing the structures of molecular sieve zeolites, and the cavity and channel systems which arise in them have been defined and described in relation to the associated molecular sieve behaviour. The electrical character of the inner surfaces of these porous crystals has been discussed, and also the nature of zeolitic fluids, in the latter case in terms of the results of a number of experimental st