Introduction The development of the pre-stressed concrete form of construction for pressure vessels has made rapid strides in recent years. Vessels of this type possess obvious advantages for nuclear power stations, and those currently under construction in this country employ such vessels for the reactor containment. During the construction of one station — for which this Company was appointed Inspecting Authority — an unusual corrosion problem arose which may well be pertinent to other pre-stressed concrete structures. The post-tensioning method of pre-stressing using tendons stretched through the concrete and anchored at the outer face of the vessel was employed. About nine months after installation of the tendons in the bottom caps of the two vessels, it was discovered that serious corrosion had occurred. The C.E.G.B. subsequently set up a Working Party on which all interested bodies were represented, in order to look into the causes and recommend preventative measures. Extensive investigations were made both on site and in the laboratory. The following briefly summarises the case and the investigations carried out.