This work deals with the thermal behaviour of medical grade silicone oils for intraocular use and of other silicones studied for comparative purposes. DSC curves of silicone oils show 1 or 2 endotherms in the range between − 43 and − 36 °C, an intermediate effect around 290 °C and a end thermal effect in the pyrolytical range 408–432 °C. The higher the MW and the viscosity, the higher the temperatures of the above thermal effects. The effects of recooling and ageing on silicone oils cause changes in the DSC curves that we have interpreted involve an enrichment in low MW components.