The optical constants n (refractive index) and k (extinction coefficient) of CVD (chemical vapor deposition) diamond thin films are important parameters not only for their optical applications but also provide information concerning the microstructure of the films. Values of n and k as well as the rms surface roughness σ have been extracted from experimental data on transmittance and reflectance for a series of CVD diamond thin films in the visible/UV and infrared ranges and are presented here. The bonded hydrogen contents N(C-H) of these diamond films have also been determined from infrared spectroscopy. A method to effectively extract a weak infrared absorption signal out of a spectrum which is dominated by strong interference fringes has been demonstrated. The changes of n and k as well as of N(C-H) as functions of the reactant gas ratio CH 4 /H 2 are presented. The effective medium approximation is applied to these films in order to provide information concerning film microstructure. Results of SEM, AFM, and Raman spectroscopy are also presented. The correlation between film quality, optical constants, hydrogen content, and volume fractions of diamond and non-diamond components for CVD diamond films has been shown. The analysis indicates that the bonded hydrogen in the films is associated primarily with the inner surface of voids.