The results of fluorescence studies on phenanthrene single crystals and on mixed crystals of phenanthrene with anthracene or tetracene as guest are the following: Even after extreme zone refining phenanthrene crystals contain impurities which have a strong influence on the fluorescence spectra. The intrinsic fluorescence spectrum of phenanthrene is analyzed with the O.O-band at 28 590 cm-1. The quantum yield is of the order 0.1 and temperature dependent. The quantum ratios for the guest and host emission in the mixed crystals can be explained only partially by an exciton diffusion mechanism. There is a partial quenching of energy transfer around 60°K. The low quantum efficiency and its dependence on temperature and concentration prevent a complete analysis of the energy transfer measurements.