Abstract: A method for the production of azacycloalkane-2,2-diphosphonic acids of general formula; ##STR1## where n is a whole number of from 2 to 16, Y is H, an alkyl or alkenyl, a substituted alkyl, or an aryl, such that the group "Y" remains unchanged during the reaction,by reaction of a lactam with a phosphorus trihalide in the presence of phosphorous acid and/or water, in which the reaction mixture also includes an organic amine and/or or an organic amine salt, said reaction being followed by hydrolysis of the reaction mixture and purification of the reaction product to obtain the diphosphonic acid.The method is exemplified by the preparation of azacycloheptane-2,2-diphosphonic acid, and azacyclononane-2,2-diphosphonic acid.