The complexity of composite spectra of close binary star system makes study of the spectra of their component stars extremely difficult . For this reason there exists very little information on the photospheric chemical composition of stars in close binaries , despite its importance for informing our understanding of the evolutionary processes of stars . In a long-term observational project we aim to fill this gap with systematic abundance studies for the variety of binary systems . The core of our analysis is the spectral disentangling technique , which allows isolation of the individual component star spectra from the time-series of observed spectra . We present new results for high-mass stars in close binaries . So far , we have measured detailed abundances for 22 stars in a dozen detached binary systems . The parameter space for the stars in our sample comprises masses in the range 8–22 M _ { \odot } , surface gravities of 3.1–4.2 ( c.g.s . ) and projected rotational velocities of 30–240 km s ^ { -1 } . Whilst recent evolutionary models for rotating single stars predict changes in photospheric abundances even during the main sequence lifetime , no star in our sample shows signs of these predicted changes . It is clear that other effects prevail in the chemical evolution of components in binary stars even at the beginning of their evolution .