Magneto-transmission measurements were carried out on a series of ZnMnSe/ZnBeSe quantum wells (QWs) in order to determine the band offset of ZnBeSe relative to ZnSe. We observed strong transitions from the ground state of the valence hand to that of the conduction band in this system. The transition energy was observed to systematically increase with increasing Be concentration in the ZnBeSe barrier. Transitions also exhibited a large Zeeman shift due to the presence of Mn in the well layer. We calculated transition energies and their magnetic field dependence for this system using the 8-band k . p model, in which the valence band offset (VBO) was the only fitting parameter. By comparing the experimental results with the calculations we conclude that the chemical VBO between ZnSe and ZnBeSe corresponds to approximately 40% of the band gap difference.