Gold-copper nanocrystalline alloys were electrodeposited onto gold substrates from a gold-copper alkaline cyanide bath by direct current plating. The electrochemical behaviour of the bath was studied by cyclic voltammetry for temperatures between 20 and 80 degrees C. An increase in the temperature of the plating bath as well as the reduction potential result in an increase in copper content of deposits. The surface morphology of the deposits was observed by scanning electron microscopy. Smooth, columnar and nodular coatings were obtained as a function of copper content and thickness of the deposit. The average grain size determined by X-ray diffraction of Au-Cu alloys varied from 3 to 35 nm and was found to decrease with the amount of copper in the coating. Au-Cu alloy containing 12 wt % Cu and having a grain size of 6 nm exhibited the maximum microhardness of 378 HV. The alloys with grain sizes in the range 6-35 nm, followed a direct Hall-Petch relation, i.e. microhardness increased with a reduction in the grain size. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3554727] All rights reserved.