  The complexity of composite spectra of close binaries makes the study of the
individual stellar spectra extremely difficult. For this reason there exists
very little information on the chemical composition of high-mass stars in close
binaries, despite its importance for understanding the evolution of massive
stars and close binary systems. A way around this problem exists: spectral
disentangling allows a time-series of composite spectra to be decomposed into
their individual components whilst preserving the total signal-to-noise ratio
in the input spectra. Here we present the results of our ongoing project to
obtain the atmospheric parameters of high-mass components in binary and
multiple systems using spectral disentangling. So far, we have performed
detailed abundance studies for 14 stars in eight eclipsing binary systems. Of
these, V380 Cyg, V621 Per and V453 Cyg are the most informative as their
primary components are evolved either close to or beyond the TAMS. Contrary to
theoretical predictions of rotating single-star evolutionary models, both of
these stars show no abundance changes relative to unevolved main sequence stars
of the same mass. It is obvious that other effects are important in the
chemical evolution of components in binary stars. Analyses are ongoing for
further systems, including AH Cep, CW Cep and V478 Cyg.
