Abstract: Possible emission of hydrogen sulphide from building materials incorporating a sulphide-containing constituent, for example ground blast furnace slag, can be suppressed or prevented by the application of a surface covering composition containing gypsum plaster and a metal compound capable of reacting with hydrogen sulphide to form a stable sulphide. It is preferred that the metal compound should have no substantial effect on the setting rate of the gypsum plaster, and may for example be an insoluble compound, while it is also preferred that the sulphide should be colorless and insoluble. Preferred metal compounds are zinc compounds and especially zinc oxide. Surface covering compositions for the purposes of the invention may contain from 0.01 to 1% by weight of the metal compound. The sulphide-containing constituent may be in a building structure or may itself be contained in a surface covering composition of rendering.