A rapid, accurate method for thorium is proposed in which thorium is titrated with a standard solution of ethylenediaminetetraacetic acid (EDTA). Alizarin Red S serves as the indicator, a sharp change from pink to yellow marking the end point of the titration. The method is selective for thorium although several cations and anions interfere. A preliminary extraction of thorium nitrate by mentyl oxide provides an excellent separation of thorium from most interfering ions.