A comparative morphology, grain structure and optical properties studies of reactive electron beam co-evaporated mixed thin films of hafnia-silica (HfO2:SiO2) and zirconia-silica (ZrO2:SiO2) systems have been carried out using atomic force microscopy and phase modulated ellipsometry. The addition of silica, especially with small fractions, has demonstrated altogether different types of evolutions in the microstructure and morphology in these binary thin film systems possibly forming new solid solution phases. Such morphological evolutions are probed through RMS roughness, power spectral density, height-height correlation and autocorrelation analyses of the topographic data acquired through atomic force microscopy. The present investigations indicated that with composition-control morphological quality improvement is more favourable in composite hafnia-silica over the zirconia-silica films. So for ultraviolet optical coating applications which demand low light scattering thin film microstructure, hafnia-silica binary composite system has a definite edge over the zirconia-silica, counterpart. (C) 2007 Elsevier B.V. All rights reserved.