A detailed electrochemical study of the ternary Cu + Ag + Se system on Pt substrate using the techniques of cyclic voltammetry (CV), hydrodynamic voltammetry at Pt-rotated disc electrode (RDE), anodic stripping has identified optimum conditions for the electrodeposition of ternary CuAgSe thin semiconductor films used as active membranes in developing Cu(II)-ion-selective sensors. A combined coulometry-quartz crystal microbalance (EQCM) protocol has been developed which allows real-time phase composition analysis of the target ternary CuAgSe and admixture with either Cu2Se or Ag2Se binary selenides. The EQCM data have provided valuable information concerning the mechanistic details of the CuAgSe-electrodeposition. The molar ratio of Cu:Se in the electrolytic bath is shown to be of crucial importance for depositing device quality films. Different mechanisms dominate when this ratio is increased from 1:5 to 1: 1, from one-step diffusion-controlled electrochemical process (Cu:Se = 1:5) to a mixed electrochemical and chemical pathway for Ag+ assimilation (Cu:Se = 1: 1). (C) 2005 Elsevier Ltd. All rights reserved.