We grew ZnO and Zn1-xMgxO thin films on (0001) sapphire substrate by metal-organic vapor phase epitaxy and measured the dielectric functions using variable angle spectroscopic ellipsometry. We analyzed the dielectric functions using a multi-layer model. We included non-uniform thickness of layers in the model. The dielectric functions were fitted using the Holden model dielectric function. We used anisotropic layer modeling for the ZnO thin film, whereas we adopted the approximation of isotropic layer modeling for Zn1-xMgxO alloys. We also discussed the Mg composition dependence of the band-gap and binding energy in Zn1-xMgxO alloys. (C) 2003 Elsevier B.V. All rights reserved.