The dynamics of photoisomerisation and rotational relaxation of 3,3'-diethyloxadicarbocyanine iodide (DODCI) in room temperature ionic liquid (IL) and binary mixture of IL and water are investigated using steady state and time resolved fluorescence spectroscopy. The photoisomerisation rate of DODCI is drastically retarded in room temperature IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) compared to isopolar solvent. The rotational relaxation time of the same dye is much slower in IL compared to the isopolar conventional solvent and become faster in presence of traces amount of water. The activation energy for photoisomerisation in different media (IL and IL-water) is also determined. (C) 2004 Elsevier B.V. All rights reserved.