[en] The source stock composition is a basic parameter for LiNbO₃ monocrystal growth. Chemical analyses do not allow to determine the composition with a sufficient accuracy, and a number of physico-chemical methods can be applied only to monocrystal specimens. For testing the Lisub(x)NbOsub(2.5+x/2) specimen composition a method has been used, based on a linear dependence of 153 and 238 cm-¹ band half-widths of the IR spectrum on the composition in the solid solution range, where x varies within the 1.0-0.9 limits. Experiments were carried out with mono- and polycrystal specimens. The accuracy of 0.01-0.005 is attained for determination of x of the composition. The analysis time is 15-20 min. Any Raman spectrometer and a laser of > 50 mW power can be used in analysis