This paper reports a detailed structural study on the nucleation of t-HfO2 nanocrystals in thin films of 70 SiO2 - 30 HfO2 prepared by sol-gel route on v-SiO2 substrates. Thermal treatment was performed at different temperatures ranging from 900 to 1200 °C for short (30 min) or long (24 h) time periods. Crystallisation and microstructure evolutions were traced by X-ray diffraction (XRD). The local structure around hafnium ions was determined from Hf L3-edge extended X-ray absorption fine structure (EXAFS) measurements carried out at the BM08-GILDA Beamline of ESRF (France). XRD shows the nucleation of HfO2 nanocrystals in the tetragonal phase after heat treatment at 1000 °C for 30 min, and a partial phase transformation to the monoclinic phase (m-HfO2) starts after heat treatment at 1200 °C for 30 min. The lattice parameters as well as the average crystallites size and their distributions were determined as a function of the heat treatment. EXAFS results are in agreement with the XRD ones, with hafnium ions in the film heat treated at 1100 °C for 24 h are present in mixed phases. © 2006 Elsevier Ltd. All rights reserved.