A method for quantitative analysis of multicomponent mixtures with unknown individual spectra of the components, based on resolution of overlapping bands, is developed and its reliability is illustrated in analysis of both model and real mixtures. The two-step optimization procedure allows attainment of high precision, the molar parts and the individual spectra of the components. This approach could be used in molecular spectroscopy (UV-Vis-NIR, IR, Raman and CD spectroscopy) as well as in chromatography.