Zn1-xMnxSe/GaAs (100) epilayers were grown using a hot-wall epitaxy method. The spectroscopic ellipsometry was used to determine the optical dielectric constant. The obtained pseudodielectric function spectra revealed the distinct structures at energies of E-0, E-0 + Delta(0), E-1, E-1 + Delta(1), E-2 and E-0(') + Delta(0) critical points (CPs) at lower Mn composition range. These critical points were determined by analytical line-shapes fitted to numerically calculated derivatives of their pseudodielectric functions. The peak characteristics were changed with the change in Mn composition. The spectral dependence of pseudodielectric function was used to obtain the fundamental energy gaps E-0 including a unique relation with Mn composition. Also, the shifting and broadening of the CPs were observed with increasing Mn composition. (c) 2005 Elsevier B.V. All rights reserved.