This Technical Report provides a discussion of the Parameters that have a bearing on the internal corrosion of solar water heating Systems. The following topics are not dealt with in this Technical Report : - Problems of compatibility between polymeric materials (plastics and rubber) and fluids; - corrosion risks concerning the enclosure and the external surface of the absorber; - safety and health questions, especially the toxicity of heat-transfer fluids. In many fields, the corrosion Problem is hard to deal with, because it overlaps several matters. As far as solar Systems are concerned, corrosion prevention cannot be treated only in respect of a component, or only as a durability Problem, or only as a design Problem. lt cannot be solved only by specific tests, or only by design recommendations. This Technical Report addresses the question of which requirements are necessary, to predict with confidence long failure-free lifetimes in active solar Systems, from the Point of view of internal corrosion. lt gathers information provided in previous Papers on this subject Iespecially bibliography references 2, 3 and 4) while staying in agreement with them