A simple and selective method for sorption-photometric determination of thorium (IV) micro amounts in clay using a new chelating sorbent, a copolymer of maleic anhydride with styrene modified with N1,N1,N2-triphenylguanidine, is proposed. The sorbent is white substance, insoluble in water, acids, weak alkalis and organic solvents. Sorbents were first ground in an agate mortar and sieved through a sieve (0.14 mm). The effect of pH of the medium, the ionic strength of the solution, contact time of the phases, concentration of thorium (IV) in the solution and on the sorption was studied under static conditions. Thorium concentration was determined by the photometric method using 2,2’,3,4-tetrahydroxy-3’-sulfo-5’-chloro-benzene. The concentration of thorium (IV) was calculated using a calibration curve, the results were processed statistically. The full static sorption capacity of the resulting sorbent on K+ ion was 13 mmol/g. An emphasis was also made on the study of the effect of the kinetic properties of sorbents on the sorption process. The maximum degree of thorium extraction by the sorbent was achieved from the solutions with pH 4. The time dependence of sorption was also studied. The results of the study showed that the sorption equilibrium is achieved after 2.5 h of contact of the sorbent with the metal. The sorption of thorium increases with an increase in the thorium concentration in the solution and attains maximum at a concentration of 8 × 10–3 M (pH 4, = 8 × 10–3 M, Vgen = 20 ml, mabsorbed = 0.05 g, sorption capacity = 348 mg/g). The degree of thorium (IV) extraction in optimal conditions exceeds 95%. It is shown that the main micro- and macro-components of clay do not affect the results of the determination of thorium. The developed technique was applied to determine thorium in a clay sample from the Shemakhi region of Azerbaijan. The correctness of the obtained results was confirmed by the spike test.