  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_sun, surface gravities of 3.1--4.2 (c.g.s.) and projected rotational
velocities of 30--240 km/s. 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.
