Abstract: Pyridine N-oxide and montmorillonite form interlamellar complexes with basal spacings of 14·7 and 21 Å. The former corresponds to a monolayer with a tilted orientation of the pyridine ring; the latter comprises a double layer of molecules between the clay sheets. Infrared spectra show that pyridine N-oxide bonds through the NO group; in the 21 Å complexes it is coordinated to the exchangeable cations through water molecules acting as bridges which themselves are directly coordinated to the cations. In the 14·7 Å complexes a splitting of the ν(NO) band occurs with the more polarizing cations Mg and Ca; the two bands correspond to pyridine N-oxide molecules directly coordinated to the cation and to other molecules coordinated through bridging by water molecules. Cs montmorillonite forms complexes with a basal spacing of approximately 14·7 Å which also shows a splitting of the ν(NO) band.