A hysteresis phenomenon was observed in the temperature dependence of the phosphorescence spectra of 4-hydroxy-3-methoxybenzaldehyde (vanillin) in ethanol. Phosphorescence first appeared at 115 K when the temperature (T) was decreased from room temperature to 77 K, whereas it lasted until 155 K as T was raised from 77 K. Such hysteresis arises from the supercooling of ethanol and is a rather general property of ethanol solution. Moreover, the shape and position of the phosphorescence spectra changed variously in the T range of 77-155 K. The unusual spectral changes were explained on the basis of molecular geometries and excitation energies of vanillin calculated by semiempirical molecular orbital methods. Furthermore, two conformations of vanillin were deduced, which give rise to room-temperature phosphorescence for a filter paper and PVA substrate.