The complexation behavior and co-electrodeposition mechanism of Au-Sn alloy in a highly stable non-cyanide bath were studied by electrochemical analysis and quantum chemical calculation based on density functional theory (DFT). The interactions between metal ions and multiple complexing agents were revealed, and the mechanism on the high stability of the Au-Sn bath was clarified. The complexing agents potassium pyrophosphate (K4P2O7) and ethylene diamine tetraacetic acid (EDTA) have a synergetic complexation to Sn ions, which exist in one valence state of Sn2+ to form three complex ions, i.e., [Sn(P2O7)](2-), [Sn(P2O7)(2)](6-) and Sn-EDTA; while the complexing agents of 5,5-dimethylhydantoin (DMH) and sodium sulfite (Na2SO3) have a complexation to Au ions, which exist in two different valence states (i.e., Au3+ and Au+) to form two complex ions of [Au(DMH)(4)](-) and Au[(SO3)(2)](3-). Moreover, the existence of [Au(DMH)(4)](-) inhibits the decomposition of [Au(SO3)(2)](3-). Meanwhile, EDTA participates in some form of coordination with Au+, inhibites the decomposition of [Au(SO3)(2)](3-) and facilitates the reduction of [Au(SO3)(2)](3-). The co-electrodeposition of Au-Sn alloy is a diffusion-controlled process, and the nucleation of Au-Sn alloy is a progressive nucleation process. The antioxidant catechol significantly increases the overpotential of Au-Sn co-electrodeposition and improves the brightness of electrodeposited films of Au-Sn alloys. (C) The Author(s) 2018. Published by ECS.