Abstract Cu–In electrodeposited layers were annealed using rapid thermal processing (RTP) in a reactive atmosphere containing sulfur vapors. The CuInS 2 formation mechanism during sulfurization of electrodeposited precursors proceeds mainly through direct sulfurization of the metallic Cu–In alloy, forming spinel CuIn 5 S 8 and chalcopyrite CuInS 2 ternary phases. During the heating step, the Cu–In metallic alloy gets richer in copper as the temperature increases and transforms from CuIn 2 to Cu 11 In 9 , then Cu 16 In 9 and finally to Cu 7 In 3 . The use of rapidly cooled samples stopped after different durations of the process along with ex-situ XRD analysis enabled us to differentiate the Cu 16 In 9 and Cu 7 In 3 phases. Finally, the efficiency of the solar cells made with the two-step electrodeposition and RTP low-cost process reaches 11% (active area 0.421 cm 2 ), which is close to the results obtained for cells made with PVD precursors.