Аннотация:By using high-resolution spectra and a specially modified model-atmosphere code, we have obtained the main characteristics and chemical composition of the optical component of Cyg X–1. We find a metallicity ([M/H] = 0.34 dex) typical of young stars. The composition has been affected by matter transformation by the reactions of the CNO cycle at the main sequence stage ([N/C] = [N/O] = 0.7 dex) and by light-element burning ([Ne/H] = [Si/H] = 0.7 dex).