Hydrate additives can be used to mitigate hydrate formation conditions, promote hydrate growth rate and improve separation efficiency. CO2 + N-2 and CO2 + CH4 systems with presence of sodium dodecyl sulfate (SDS) or tetrahydrofuran (THE) are studied to analyze the effect of hydrate additives on gas separation performance. The experiment results show that CO2 can be selectively enriched in the hydrate phase. SDS can speed up the hydrate growth rate by facilitating gas molecules solubilization. When SOS concentration increases, 'split and loss fraction increase initially and then decrease slightly, resulting in a decreased separation factor. The optimum concentration of SDS exists at the range of 100-300 ppm. As THF can be easily encaged in hydrate cavities, hydrate formation condition can be mitigated greatly with its existence. Additionally, THF can also strengthen hydrate formation. The THE effect on separation performance is related to feed gas components. CO2 occupies the small cavities of type II hydrate prior to N-2. But the competitiveness of CO2 and CH4 to occupy cavities are quite fair. The variations of split fraction, loss fraction and separation factor depend on the concentration of THF added. The work in this paper has a positive role in flue gas CO2 capture and natural gas de-acidification. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.