ABSTRACT
	Tarnishing of copper surfaces is defined here primarily as the formation of copper sulphides, which are aesthetically and/or technologically undesirable in many areas of industry and commerce, such as in electronics, jewellery and dentistry.  Many of the publications dealing with tarnish testing do not cite the large body of scientific knowledge describing the solid-state properties and behaviour of copper sulphides, the role of chemical- and photo-catalysis in the gas phase sulphur-water-oxygen system, the importance of mass transport in gas phase tests and aqueous environments. These will be considered here, laying emphasis on the electrochemical behaviour of the tarnish process in terms of coupled anodic and cathodic reactions, the chemistry/electrochemistry of the sulphur/water system, the mechanism of formation of the copper sulphide phases.