Photoisomerization quantum yields (Phi(E-Z)) of phenylazo-substituted imidazolium cations, which are counterparts of recently developed photochromic ionic liquids, were measured under S-1(n, pi*) excitation. The Phi(E-Z) values are lower in imidazolium-type ionic liquids than those in conventional solvents and in tert-alkylammonium-type ionic liquids. This observation was argued based on solute solvent complex formation at imidazolium rings.