An automatic spectrotitration procedure for EHDP has been developed. It is applicable to samples containing 50mug or more of EHDP, with a relative standard deviation of 3% if no interfering materials are present. The sensitivity can be extended down to about 5mug, but titration blanks become significant and the standard deviation increases markedly. Thorium diaminocyclohexanetetra-acetate (Th-DCTA) is used as the titrant, to give a ternary species (Th-DCTA)(2)-EHDP. The end-point in the spectrotitration is due to deprotonation of Xylenol Orange indicator when it is incorporated into a weaker ternary species after all the EHDP has reacted. Salts such as NaCl, NaClO(4), Na(2)SO(4) and NaNO(3) interfere if present at concentrations of 0.1-0.5M. The method is much more sensitive to orthophosphate, but this interference can be averted by isolating the EHDP before titration by selectively adsorbing the phosphonate on calcium hydroxyapatite.