Carboxy-alkyl phenylphosphonic monoesters are obtained very easily by selective hydrolysis (in alcaline medium) of the corresponding carbamido- or cyano-alkyl phenylphosphonic monoesters and sometimes of the corresponding carbethoxy-alkyl phenylphosphonic monoesters and sometimes of the corresponding carbethoxy-alkyl phenylphosphonic monoesters and sometimes of the corresponding carbethoxy-alkyl phenylphosphonic