The problem of boiler plant availability in power stations has become so prominent that a better understanding is essential of the causes of the fouling of heating surfaces by deposits from the products of combustion.Earlier theories based on fused ash particles, sodium sulphate bond or high dew-points have not satisfactorily accounted for various wellestablished facts of observation. The latter include the existence of a period of apparent immunity from deposits in a new boiler plant, the characteristic behaviour of the dust from pulverized fuel firing and the peculiar scale-like form of certain hard deposits occurring on economizer and boiler tubes.The authors, restricting their observations largely to economizers, have found that hard bonded deposits result from certain chemical reactions between flue dust and sulphuric acid. The type of reaction depends on the temperature of the metal parts associated with the deposits.A clear understanding of the reactions involved in the formation of these deposits indicates methods for removing them.The authors offer practical suggestions for the prevention of hard bonded scale formation in economizers. Various types of coal ash are discussed, and characteristics favouring bonded deposit formation are pointed out. The influence of the authors' proposals on plant design is considered.