High quality hexagonal CdS epilayer was grown on GaAs (1 1 1) substrates by the hot-wall epitaxy method. The crystal structure of the grown CdS epilayers was confirmed to be the hexagonal structure by X-ray diffraction pattern and scanning electron microscopy image. The optical properties of the hexagonal CdS epilayers were investigated in a wide photon energy range between 2.0 and 8.5 eV using spectroscopic ellipsometry (SE) at room temperature. The data obtained by SE were analyzed to find the critical points of the pseudodielectric function spectra, = + i , such as E(0), E(1A), E(1B), E(0'), F(1), and two E(2) structures. In addition, the second derivative spectra, d(2)epsilon(E)/dE(2), of the pseudodielectric function of hexagonal CdS epilayers were numerically calculated to determine the critical structures. Four structures, such as E(0') F(1), and two E(2) structures, from 6.0 eV to 8.0 eV were observed, for the first time, at 300 K by ellipsometric measurements for the hexagonal CdS epilayers. (C) 2011 Elsevier B.V. All rights reserved.