The ratio of excimer to molecular fluorescence intensities in both the delayed and total emission spectra of pyrene in ethanol has been measured as a function of solute concentration and temperature from 150–350°K. An analysis of the data in terms of triplet-triplet annihilation leading directly to excimer formation (process 1) or production of the lowest excited molecular singlet state (process 2) shows that the relative probabilitiesk1/k2of these processes increases from zero at low temperatures to a maximum of 1.8 at higher temperatures.