A process for preparing 1-hydroxy, ethylidene-1,1-diphosphonic acid, which comprises contacting a phosphorus trihalide such as phosphorus trichloride with glacial acetic acid in a reaction zone maintained at a temperature of from about 50.degree.C. to about 80.degree.C. while continuously removing by-product acetyl halide as formed from the said reaction zone to form a single-phase liquid reaction product, heating said reaction product to a temperature of from 90.degree.C. to 140.degree.C. and reacting therewith acetic anhydride to form a normally solid anhydrous reaction product, and hydrolyzing said product by steam treatment to recover the desired 1-hydroxy, ethylidene-1,1-diphosphonic acid and remove by-product acetic acid. The acetyl halide by-product is recovered by absorbing same in the aqueous by-product acetic acid recovered from the hydrolysis step and thereby recovering glacial acetic acid for storage and recycle to the process. The process is adapted for continuous operation.