The ordinary optical dielectric functions of pulsed-laser-deposition grown wurtzite c-plane MgxZn1-xO (0 less than or equal to x less than or equal to 0.53) thin films have been determined by using spectroscopic ellipsometry in the photon energy range from 4.5 to 9.5 eV. The dielectric functions reveal features which resemble those previously detected in uniaxial AlGaN and identified as E-1- and E-2-type band-to-band transitions with no remarkable dependence of the transition energy on Mg content x. The E-1- and E-2-type transitions for ZnO are compared with pseudopotential band-structure calculations. (C) 2003 Elsevier B.V. All rights reserved.