Optical properties related to lattice vibrations and photoluminecence of Zn-1-xBexSe and Zn-1-(x)-yBexMnySe alloys are presented. Optical phonons of the mixed crystals grown in a sphalerite phase for composition 0less than or equal toxless than or equal to0.35 and 0less than or equal toyless than or equal to0.22 have been investigated by IR spectroscopic ellipsometry in the wave number range 230-8000 cm(-1). The complex dielectric functions can be reproduced using a model with three classical damped oscillators corresponding to the BeSe-like modes and a Drude contribution from the free carriers. The zone-centre optical-phonon frequencies increase continuously with increasing Be concentration. The amplitudes of the vibrations are also increasing with Be content. (C) 2003 Elsevier B.V. All rights reserved.