Provided is a novel method of making bisphosphonic acids, e.g. risedronic acid, including the step of combining a carboxylic acid, phoshorous acid, and a halophosphorous compound in the presence of a diluent that is an aromatic hydrocarbon or a silicone fluid. When the diluent is an aromatic hydrocarbon, a inert support or ortho-phosphoric acid codiluent is advantageously included.