A few thousand of cleansers are commercially available; many of them are declared safe (by the manufacturer) for use in daily household practice, but there is no such label as safe for museum use. The cleaning of art objects is a different subject; the risk in the use of cleansers is twofold: direct contact by accident, e.g. by spraying, and through colatile solvents and vapours. The nature and properties of the various types of products and compounds are discussed. The concept of pH, the nature of acids and bases and the other typical ingredients of cleansers are presented: soaps, solvents, bleaching agents, complexing agents, flavouring and colouring matters, preservers. Each compound has to be evaluated as to the risk it carries. In general, synthetic, neutral or weakly basic cleansers are allowed in the neighbourhood of museum objects. -- CAL