Abstract There has been much interest in semiconductor nanocrystals embedded in oxides and their interesting optical and electrical properties, which can potentially be utilised in future devices. We have studied the effects of different processing parameters on the formation of Ge nanocrystals in SiO 2 prepared by ion implantation followed by heat treatment. We implanted Ge doses of between 2×10 15 and 1×10 17 cm −2 at implantation energy of 30 or 100 keV and used a range of annealing temperatures and times. The samples were investigated using transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDS), secondary ion mass spectroscopy (SIMS) and Raman spectroscopy. Ge nanocrystals were observed for doses of 3×10 16 and 1×10 17 cm −2 at 100 keV after annealing at 800 °C. Annealing of similar samples at 1000 °C yielded no nanocrystals. We believe that diffusion of oxidising species from the atmosphere is important and that this process is faster at 1000 than at 800 °C, creating Ge-rich amorphous oxides rather than Ge nanocrystals at the higher temperature. This oxidation process also explains the absence of Ge nanocrystals in SiO 2 films implanted with Ge at 30 keV after annealing. Electron beam-induced precipitation was observed in samples with amorphous Ge-rich layers under intense electron irradiation in the TEM. Accumulation of Ge at the Si/SiO 2 interface was observed in samples implanted at 100 keV and annealed at 1000 °C or higher temperatures.