Tantalum and niobium oxide optical thin films were prepared at room temperature by plasma-enhanced chemical vapor deposition using tantalum and niobium pentaethoxide (M(OC2H5)(5)) precursors. We studied the evolution of their optical and microstructural properties as a result of annealing over a broad temperature range from room temperature up to 900 degrees C. The as-deposited films were amorphous; their refractive index, n, and extinction coefficient, k, at 550 nm were n=2.13 and k < 10(-4) for Ta2O5, and n = 2.24 and k < 10(-4) for Nb2O5. The films contained a small amount of residual carbon (similar to 2-6 at.%) bonded mostly to oxygen. During annealing, the onset of crystallization was observed at approximately T-C1 = 650 degrees C for Ta2O5 and at T-C1 =450 degrees C for Nb2O5. Upon annealing close to T-1 (300 degrees C for Nb2O5 and 400 degrees C for Ta2O5), n at 550 nm decreased by less than 1%. This was correlated with the decrease of carbon content, as suggested by Fourier transform infrared spectroscopy, elastic recoil detection and static secondary ion mass spectroscopy (SIMS) results. During annealing, we observed phase transition from the 8(hexagonal) phase to the L- (orthorhombic) phase between 800 degrees C and 900 degrees C for Ta2O5, and between 600 degrees C and 700 degrees C for Nb2O5. The structural changes were also marked by silicon diffusion from the substrate into the oxide layer at annealing temperatures above 500 degrees C for Ta2O5 and above 400 degrees C for Nb2O5. As a consequence of oxygen, silicon and metal interdiffusion, the interface between the Si substrate and the metal oxide (Ta2O5 or Nb2O5) is characterized by its broadening, well documented by spectroscopic ellipsometry and SIMS data. (c) 2006 Published by Elsevier B.V.