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Photoluminescence and reflectivity measurements have been carried out on Zn 1− x Mn x Se solid solutions in the complete range of crystal compositions, 0≤ x≤ 0.55. The features of the photoluminescence and reflectivity data near the band edge enabled us to determine values of the fundamental band gap as a function of Mn molar fraction x and temperature (8 K≤ T≤ 300 K). The energy gap in crystals with compositions in the vicinity of the zinc-blende–wurtzite structural transition (0.2≲ x≲ 0.3) exhibited a scatter of values, probably associated with the presence of various polytypes (as evidenced by transmission-electron-microscopy measurements). The dependence of E g on x is anomalous for 0≤ x≤ 0.2, showing a minimum in the E g vs x curve. A simple model (in terms of second-order perturbation theory in s-d and p-d interactions) relates this minimum to a maximum observed in the magnetic …