Gold nanoclusters were formed in (100) MgO containing nanosize cavities. Nanocavities were created by a combination of 30 keV 3He implantation and subsequent annealing. Au was introduced inside the cavities by 30 keV Au ion implantation. The implantation dose varied from 1016 to 3×1016 cm−2 and the implantation temperature varied from room temperature (RT) to 1270 K. In the case of the RT implantations the samples were subsequently annealed stepwise in air up to 1370 K. The evolution of the implantation defects and the formation of nanoclusters was monitored by positron beam analysis (PBA), optical absorption spectrometry and Rutherford backscattering spectroscopy (RBS)/channeling. Optical absorption resonance peaks were observed at 570–610 nm for the RT implantations and at 525–550 nm for the implantations carried out at elevated temperatures. In the case of 1270 K implantation an extra absorption peak at 818 nm was observed. The influence of the implantation temperature on shape and position of the absorption band ascribed to nanoclusters is discussed.