The fluorescence spectra excited from anthracene crystals containing traces of tetracene have been accurately recorded using a spectrometer with a photomultiplier as detector. In addition polarized fluorescence spectra from very thin anthracene single crystals have been reported. From the results it is concluded that energy migration from anthracene to tetracene is by a non-radiative mechanism, although it is shown that reabsorption of fluorescence can significantly modify the characteristics of such crystals. In addition, it is shown that this system has a limit of solid solution for crystals grown by sublimation (about mole/mole) above which the characteristics are markedly altered because of the formation of minute tetracene crystals.