Kinetic hydrate inhibitors (KHIs) are used to prevent gas hydrate formation in gas and oilfield operations. All KHIs discovered to date are water-soluble polymers. However, their performance can be enhanced by certain non-polymeric organic molecules. Recently, it was claimed that certain imidazolium-based ionic liquids could have a dual function, acting as both thermodynamic inhibitors and KHIs (Xiao, C., Adidharma, H., 2009. Chem. Eng. Sci. 64, 1522). As the KHI experimental work was carried out at a temperature of -12 degrees C, giving a very high subcooling of about 25 degrees C, we reinvestigated two of these ionic liquids at more typical subsea temperatures and subcoolings. We find that these ionic liquids are very poor KHIs when used alone at 500010000 ppm, but they are fairly good synergists for commercial KHIs based on vinyl lactam polymers and hyperbranched poly(ester amide)s. Both ionic liquids showed only weak growth inhibition of tetrahydrofuran hydrate crystals. Finally, both ionic liquids were poorly biodegraded in the OECD306 seawater 28 day biodegradation test. (C) 2010 Elsevier Ltd. All rights reserved.