Abstract: The invention concerns a process for the preparation of diphosphonic acids starting from carboxylic acids, a phosphorus trihalide and orthophosphorous acid, without solvents, using particular molar ratios and conditions so as to keep the reaction mixture fluid and carrying out the hydrolysis only with boiling water.

Abstract: Biphosphonic acids of general formula I: ##STR1## in which R is a fluorine atom of a linear or branched alkyl radical containing between 1 and 5 carbon atoms, which may also be substituted by one or more amino groups of fluorine atoms or both amino groups and fluorine atoms, R' is hydroxy or fluorine, and their salts with an alkali metal, an organic base or a basic aminoacid, exhibit valuable properties in the treatment of urolithiasis or in the treatment as inhibitors of bone reabsorption. The compound 4-amino-1-hydroxybutan-1,1-biphosphonic acid is between 100 and 300 times more active than Cl.sub.2 MDP.