The thenoyltrifluoroacetone (TTA) extraction method has been investigated for the determination of microgram concentrations of thorium in uranium ores and concertrates. The tolerance limits for eighteen elements commonly found in uranium ore concentrates were established. The effect of phosphate on estraction of thorium was studied. Cobalt, copper, tin, and bismuth were found to seriously interfere with the Thoron method for the determination of thorium after a TTA extraction. A TTA extraction method is described for the separation of thorium and uranium from the interfering ions, employing preliminary acid sulfide and ammonium hydroxide separations. The method is rapid, and results obtained compare favorably with results obtained with other procedures. (auth).