The determination of the above band gap optical properties of zincblendeCd1−xMgxTe(0⩽x⩽0.5) ternary alloys are reported on. Using the parabolic-band critical point model, room-temperature critical point energies of theE0,E0+&Dgr;0,E1,E1+&Dgr;1,E2, andE0′interband transitions from numerically calculated second energy derivatives of ellipsometric spectra were obtained. The presence of two distinct structures in theE2feature forx>0was also observed. ©1997 American Institute of Physics.