The alkaline deacylation of a representative series of 1-(acylamino)alkylphosphonic acids [(AC)-AAP: (AC) = Ac, TFA, Bz; AAP = GlyP, AlaP, ValP, PglP and PheP] in an aqueous solution of KOH (2M) was investigated. The results suggested a two-stage reaction mechanism with a quick interaction of the hydroxyl ion on the carbonyl function of the amide R-C(O)-N(H)- group in the first stage, which leads to instant formation of the intermediary acyl-hydroxyl adducts of R-C(O−)2-N(H)-, visible in the 31P NMR spectra. In the second stage, these intermediates decompose slowly by splitting of the RC(O−)2-N(H)- function with the subsequent formation of 1-aminoalkylphosphonate and carboxylate ions.