Zn1−xMgxSe alloys have been grown on (100) GaAs substrates by molecular beam epitaxy. Their crystal structures are determined form x‐ray diffraction spectra and are found to be zinc blende, rocksalt, and their mixture forx<0.5,x=1, andx∼0.6, respectively. The Raman spectra are measured and show two‐mode behavior for those alloys with zinc blende structure. The long‐wavelength frequencies of ZnSe‐ and MgSe‐like optical phonons are determined and the results can be expressed as linear functions ofx. The first‐order Raman spectra are observed for MgSe and Mg‐rich Zn1−xMgxSe alloys with rocksalt structure. Those Raman spectra are attributed to the scattering from optical phonons activated by disorders and impurities, and their line shapes represent the phonon density of states. ©1995 American Institute of Physics.