Low-temperature UV-vis spectra of spironaphtooxazine (SNO) in a glass forming organic mixture clearly evidenced the J-aggregate formation. J-Aggregate becomes unstable above 100K and its quantum efficiency for the visible photoreaction was lower than that of monomeric open form B. In a dilute solution, the intensity of the J-band was maximized at 150K while it decreases with temperature in a concentrated solution.