We studied the photochromism of a newly synthesized ionic liquid, [2PA-Bmim]Tf(2)N ([2PA-Bmim](+): 3-butyl-1-methyl-2-phenylazoimidazolium,Tf(2)N(-): bis(trifluoromethanesulfonyl)-amide) which is characterized by a phenylazo group substituted on the imidazolium ring. The melting point of [2PA-Bmim]Tf(2)N is 329 K. The absorption spectrum of [2PA-Bmim](+) dissolved in conventional organic solvents or in ionic liquids changes drastically upon UV-light irradiation, which is attributed to the photoisomerization of the phenylazo group from E- to Z-forms during irradiation and the backward thermal isomerization from Z- to E-forms in the dark. The E-Z photoisomerization quantum yield, Phi(iso), was determined by 355 nm laser photolysis. The Phi(iso) value slightly depends on solvent viscosity, from 0.12 in 3-butyl-1-methylimidazolium PF(6)(-) (eta = 241 cP) to 0.19 in toluene (eta < 1 cP). On the other hand, no solvent dependence was observed for Arrhenius parameters of the backward Z-E thermal isomerization. We discuss the isomerization mechanism and the reason why the E-Z photoisomerization yield depends on solvent viscosity. (C) 2009 Elsevier B.V. All rights reserved.