Technical report describing that the use of controlled amounts of carbonate, acting both as complexing agent buffer led to a fairly satisfactory procedure for the determination of Th in Th-U alloys, using cupferron. The alloy, dissolved in NHO2, in the presence of HF, is treated with H2SO4 and the mixture is evaporated in SO2 fumes. The solution is diluted and (NH4)2CO2 is added. Th is precipitated from this solution with cupferron. The precipitate is weighed as ThO2 after ignition. An accuracy of 6% was obtained. A method was developed specifically for the determination of Th in U-Th alloys containing 1 to 10% Th. An ion exchange resin in column is used to separate Th from U, with NH2OH-HCl as complexing agent. The Th is then precipitated, ignited, and weighed in the conventional manner.