This review examines methods of measuring mass transfer, particularly those suitable for dissolving surfaces which can roughen. Suitability for use in high quality two phase flows is also considered. A method is described which uses the corrosion of copper in dilute hydrochloric acid containing ferric ions. The application of this technique to measure mass transfer at various geometries is described including: the region downstream of an orifice, 180° bends, seven different impinging jet geometries, annular two phase flow in pipes and bends and the development and effects of roughness.