[en] Exciton reflection spectra and their magnetic field splittings for (CdSe)_1-x(MnS)_x crystals at x=0.01, 0.05 and 0.1 are investigated. Nonadditivity of exciton energy shift with simultaneous cation and anion substitution is traced. It is found that dependence of exchange constant J_hM on sulphur concentration is weaker than that expected from linear approximation on the basis of the data previously obtained for Cd_1-xMn_xS and Cd_1-xMn_xSe. It is also stated that Se-S substitution causes a larger exciton line broadening than substitutions in cation sublattice