[en] An instrument relative method for analyzing the isotopic substance composition, which is based on using the process of the Coulomb nucleus excitation, is suggested. Compared are γ-spectra of the Coulomb excitation under the effect of heavy ions of a target being investigated and a similar target of natural isotopic composition. A formula connected the values being measured with the isotope content in a specimen under investigation is presented. The method has been tested experimentally in determining isotope composition of tungsten monocrystal enriched with ¹⁸⁶W. The reference target and target being investigated were irradiated with ⁴⁰Ar ion beams of 135 MeV. It is seen from the results obtained, that the limiting level of isotope content, which can be determined, equals about 0.5 % under concrete conditions of the experiment