Anew series of(alpha -hydroxyalkyl)phosphinic acids I was prepared in good yields from long-chain aldehydes (C-8-C-18) and 50% aqueous hypophosphorous acid under sonication. Oxidation of these phosphinic acids by dimethyl sulfoxide in the presence of catalytic amounts of iodine afforded the corresponding (oc-hydroxyalkyl)phosphonic acids 2. Critical micelle concentrations (cmc) of the amphiphilic monocaternary (alpha -hydroxyalkyl)phosphorus acid ammonium salts 1 and 2, determined by tensiometry and P-31 NMR, showed a good correlation between the two methods and indicated their high surfactant properties. The phosphonate salts 2 with two dissociable groups had much higher cmc values than those of the corresponding phosphinate analogues 1. The ammonium salts of these phosphorus acids were able to form ordered lyotropic mesophases in concentrated water solutions, visualized by optical polarization microscopy. The nature of the observed mesophases (hexagonal or lamellar) depended on the concentration, the temperature, the nature of the polar head, and the length of the alkyl chain, the hexagonal phase being promoted for shorter chains (C-8 and C-10). The ability of these acids 1-2 to form stable and compact Langmuir monolayers at the air/water interface was studied. Their conformations at the air/water interface were estimated by a molecular modeling study.