The invention relates to a process for making 1-aminoalkane-1,1-diphosphonic acids of the general formula (I): ##STR1## in which R stands for an aliphatic hydrocarbon radical having from 1 to 12 carbon atoms.To this end, the invention provides:(a) for tetraphosphorus hexoxide to be reacted with at least one compound of the general formula (II) ##STR2## in which R has the meaning given above and X stands for an --ONH.sub.4 or NH.sub.2 -radical, in inert gas atmosphere at elevated temperature, the molar ratio of tetraphosphorus hexoxide to the compound of general formula (II) being about 1 to 2-6; and(b) for 1-aminoalkane-1,1-diphosphonic acid to be crystallized from the reaction mixture.