Molten reduction to release the value metals are the necessary step in the copper slag recovery process. How to effectively choose and utilize the reducing agents has an extremely important significance. In the research, molten reductions with the CO-H-2 -Ar mixed gases are investigated. De-oxidation amounts and de-oxidation rates maintain stability first and then decrease with the increase of CO/H-2 ratios. They are all much higher than those of pure CO or H-2 due to the water-gas shift reaction. The magnetite phase decreased gradually with the decrease of CO/H-2 ratios, and some pure ferrous phase can generate in the slag as the ratios are less than 1:1. Otherwise, the apparent rate constants, the reaction orders and the activation energies for the various CO/H-2 ratios are also measured. The results illustrate that the reaction process will vary with the variety of mixed gas compositions. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.