State-of-the-art superalloys are useful for high temperature applications, in large part, because they form protective alumina surface films by the selective oxidation of aluminum from the alloy. The adherence of the alumina to the alloy is crucial to maintaining oxidation resistance, particularly under thermal cycling conditions. It is now well established that small additions of reactive elements, such as yttrium, hafnium, and cerium, substantially improve the adherence of alumina films to alloy substrates. While the effects produced by the reactive elements are widely known the mechanisms whereby they improve adherence are nor completely understood. Over the last fifty years a number of mechanisms have been proposed. However, it has recently become clear that a major effect of the reactive elements is to tie up sulfur in the alloy and prevent it from segregating to the alloy/oxide interface and weakening an otherwise strong bond.