[en] An XRF method for the determination of K, Rb and Cs in thoria has been developed using Philips PW-1220 semiautomatic X-ray spectrometer. Thoria samples are mixed with boric acid binder in proportion of 900 mg: 100 mg and the mixture is pressed over a boric acid backing pellet to form a double layer pellet. Tungsten target X-ray tube for excitation and LiF(200) crystal for dispersion have been used. The minimum determination limits of K, Rb and Cs are 100, 50 and 50 ppm respectively. (author)