Photochemical photon upconverters based on triplet-triplet annihilation (TTA) between excited molecules have been developed using ionic liquids (ILs) for the first time. The employed aromatic molecules are found to be stable in the polar environment of ILs, contrary to expectation. The mechanism of the stable solvation is proposed. The upconversion quantum yields (UC-QYs) for different ILs are measured and it is found that the UC-QYs rapidly saturate as the excitation light power increases. An analytical model was developed and compared with the experimental data. It is shown that ILs are not viscous media for the purpose of TTA-based upconversion. (C) 2011 Elsevier B.V. All rights reserved.