Here are presented the results of laboratory studies on various types of acid rhodium plating electrolyte, with particular reference to sulphate and phosphate-sulphate solutions, but including exploratory trials on phosphate, fluoborate and sulphamate electrolytes. The sulphate electrolyte is preferred for general purposes. For the first two electrolytes, examination is made of the effects of variations in preparative procedure on the characteristics of electrolytes of the same nominal composition, and the influence of composition and deposition factors on the internal stress of deposits. Factors affecting the incidence of cracking of thick rhodium deposits are discussed, and on the basis of laboratory and field experience, practical recommendations are made concerning the formulation of sulphate electrolytes and the optimum operating conditions for the production of rhodium deposits in the thickness range 0·002-0·002” from this type of solution. Procedures are detailed for either the rhodium or silver/rhodium plating of the basis metals most commonly involved in industrial applications.