Abstract: A method for the preparation of alkene phosphonic acids and salts thereof, such as VDPA (vinylidene diphosphonic acid) is disclosed. The products can be produced in a stable, substantially pure form and in high yield. The method includes the azeotropic removal of water from salts of &agr;-hydroxy-alkane diphosphonic acid dimer or corresponding acids thereof, and the pyrolysis of the dehydrated reactant at a temperature of from 170° C. to 300° C. There may be included a step to convert anhydrides which have been formed during the pyrolysis. The method may include the use of heat transfer agents and/or bases, and may be carried out at elevated or reduced pressure.