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Complexes formed between montmorillonite, saturated in Li+, Na+, Mg2+, Ca2+, Co2+, Fe3+, Cu2+and Zn2+, and trimethyl phosphites (TMP) and triethyl phosphites (TEP) were studied. In all of the cases, phosphites penetrate into the interlayer space of the montmorillonite and produce solvates whose basal spacing varies depending on the characteristics of the exchangeable cation. AU the complexes with low basal spacing (Li+, Na+, Mg2+, Co2+and Zn2+) are stable in vacuum, whereas those with high basal spacing, formed by the Ca2+sample with TMP, and Ca2+and Fe3+samples with TEP are transformed into low basal spacing complexes in vacuum. The complexes with high basal spacing (Cu2+sample with TMP and TEP) are stable in vacuum. The TMP and TEP complexes stable in vacuum with low spacing are thermally destroyed in one or two stages with two loss maxima, as a result of partial burning of phosphite molecules. Those with high spacing (Cu2+) are destroyed in two stages; the first is probably the result of the transformation process from high to low spacing, as a consequence of the structural reorganization of the molecules which remain in the interlayer space, and the second, could be associated with the destruction of low spacing complexes. The IR spectra show that the molecule and the cation are linked by the P of the phosphite, which produces a reinforcement of the other bonds in the molecule, caused by an inductive effect. The phosphite intercalation is accompanied by a partial isomerization of phosphite to phosphonate. The heat of adsorption of phosphites shows that the molecule-cation bond is ion-dipole. In the Cu sample with trimethyl phosphite, this bond seems to be reinforced by retrodonation of electrons from copper to ligand. Finally, the possible disposition of phosphite molecules in the interlayer space is considered. For this purpose, ab initio calculations have been performed on the different conformers of the TMP molecule at 6–31G* and 6–31+G* basis sets.