Abstract: Phenomena at the interface of solids and the insulating gas are of high interest since they are responsible for changes finally leading to a reduction of the insulating ability in this 'weak' zone. Depending on the type of stresses of electrical as well as of non- electrical nature and on the type of the solid and of the insulating gas, specific ageing mechanisms have to be considered. Recent investigations have focused on the so-called 'early stage of ageing' and the physical and chemical effects responsible for the attack of the surface of the organic solid insulators. For the study of the 'early stage of ageing' various ageing procedures and diagnostic tools have been developed and applied and numerous model post insulators made of epoxy and polyurethan resins with different types of mineral fillers have been tested. A model of surface ageing could be developed
Key words: Surface phenomena, indoor isulation, outdoor insulation, early stage of eaging, diagnostic tools.