  A significant fraction of massive stars are found in multiple systems. The
effect of binarity on stellar evolution is poorly constrained. In particular,
the role of tides and mass transfer on surface chemical abundances is not
constrained observationally. The aim of this study is to investigate the effect
of binarity on the stellar properties and surface abundances of massive
binaries. We perform a spectroscopic analysis of six Galactic massive binaries.
The spectra of individual components are obtained from a spectral disentangling
method and are subsequently analyzed by means of atmosphere models. The stellar
parameters and CNO surface abundances are determined. Most systems are made of
main-sequence stars. Three systems are detached, two are in contact and no
information is available for the sixth system. For eleven out of the twelve
stars studied the surface abundances are only mildly affected by stellar
evolution and mixing. They are not different from those of single stars, within
the uncertainties. The secondary of XZ~Cep is strongly chemically enriched.
Considering previous determinations of surface abundance in massive binary
systems suggests that the effect of tides on chemical mixing is limited,
whereas mass transfer and removal of outer layers of the mass donor leads to
the appearance of chemically processed material at the surface, although this
is not systematic. The evolutionary masses of the components of our six systems
are on average 16.5% higher than the dynamical masses. Some systems seem to
have reached synchronization, while others may still be in a transitory phase.
