Alloys which are exposed to oxidizing atmospheres at high temperatures are protected from catastrophic oxidation by the oxide layer,or layers,which form on their surface.The relevant oxides in this case are chromia and alumina.In order to be protective it is essential that these oxide layers adhere well to the underlying metal under thermal cycling conditions.Seventy years ago small amounts of certain elements were found to improve the adhesion significantly.These elements,which are known as reactive elements,include cerium and yttrium.Various hypotheses were proposed to explain their beneficial effect but no one hypothesis explained all the results.More recently their effect has been attributed to their reaction with impurity sulphur which,when the reactive element is not present,segregates to the oxide/metal interface and significantly weakens the adhesion.The reactive element prevents this segregation from occurring and hence improves the adhesion.This,together with the effect of the reactive elements on the scale growth-mechanism,has been called "The Sulphur Effect".The present state of this hypothesis will be reviewed and its relevance to the great improvement of oxide/metal adhesion produced by nanocrystallinity will be discussed.