The n-decanephosphonic acid/water and n-dodecanephosphonic acid/water systems were studied by differential scanning calorimetry, hot stage polarized light microscopy, and FT-IR spectropetry. There is a rich polymorphism : in both systems there are three liquid crystalline phases (lamellar L(alpha) and L(beta) and an inverse cubic phase) plus the isotropic Liquid and two solid phases. One of the solids is an ordinary cristalline phase and the other is a waxy crystal in which the polar headgroup network is rigid and ordered but in which the alkyl chains are partially fluid and disordered. The water in these systems exists in two distinct forms : water bound to the headgroups and water which is effectively free. The polymorphism of the heptyl and octyl analogues has been studied by Klose et al.