Beneficiation of zinc from low-grade oxidized zinc ores has become the focus of recent studies. Sulfidation is an important process to recover zinc from oxidized zinc ores in flotation. Although some studies have reported on the sulfidation-flotation of smithsonite, the effect of sulfide ions on sulfide film thickness remains unknown when the oxidized zinc ores start floating and achieve excellent flotation. In this work, the influence of sulfur ions on smithsonite flotation in a dodecylamine system were investigated by micro-flotation experiments, inductively coupled plasma-atomic emission spectrometry (ICP-AES), zeta potential measurements, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Micro-flotation experiment results show that smithsonite starts floating after treatment with a low S concentration of 5 x 10(-5) mol/L, while excellent flotation was obtained following treatment with a high S concentration of 5 x 10(-4) mol/L. Compared to the low S solution, excellent hydrophobicity of smithsonite was obtained owing to the adsorption of more sulfide ions, making the mineral surface potential more negative, and forming a thicker heterogeneous zinc-sulfide film that primary consists of zinc monosulfide and zinc polysulfide. The zinc-sulfide film thickness on the smithsonite surface was approximately 7.79 nm when the smithsonite started floating, while excellent floatation was obtained as the zinc-sulfide film thickness reached about 12.13 nm. (C) 2019 Published by Elsevier. B.V.