A process for the prepn. of biphosphonic acids, (HO)2P(O)C(R1)(OH)P(O)(OH)2 (R1 = alkyl, arylalkyl, arom., heteroarom.), and pharmaceutical acceptable salts thereof, comprises reacting a carboxylic acid with phosphorous trichloride and phosphorous acid the presence of aprotic polar solvent. Thus, reaction of 3-pyridylacetic acid with H3PO3 N,N'-dimethylethyleneurea at 40-50 followed by treatment with PCl3 and adjusting pH to 8-9 via treatment with NaOH gave 60% risedronic acid sodium salt.