The spectral and photophysical properties of trans-1-pyrazinyl-2-(3-quinolinyl)ethylene (trans-3- PyQE) are investigated under various conditions in order to obtain information on ground and excited states. The absorption spectrum of trans-3-PyQE changes slightly with varying degree of solvent polarity ; the. fluorescence spectrum is shifted to the red and becomes broad and structureless as the solvent polarity increases. The fluorescence quantum yield increases with increasing solvent polarity. The fluorescence intensity of trans-3-PyQE decreases as the concentration of methyl iodide increases. The fluorescence spectra of trans-3-PyQE changes markedly upon the variation of the excitation wavelength, presumably due to an equilibrium between conformers originating from the rotation of a quasi-single bond between the quinolinyl group and ethylenic carbon atom. These results indicate that the spectral and photophysical properties of trans-3-PyQE are strongly influenced by solvent, heavy atom, and an equilibrium between conformers