The origin of the red-shifted fluorescence emission spectra observed for commercial zone-refined phenanthrene (purity 99.5+%) in concentrated cyclohexane solution and as sorbates on porous silica at high loading levels was studied by absorption and fluorescence spectroscopy as well as fluorescence quenching measurements, The absorption and fluorescence spectra indicate that a small amount of anthracene is present in the zone-refined phenanthrene. In concentrated phenanthrene solution and highly phenanthrene-loaded porous silica, excitation energy transfer from phenanthrene to the anthracene: impurity results in the reduction of the phenanthrene fluorescence intensity and the appearance of the anthracene fluorescence emission at longer wavelength. A Stern-Volmer plot for quenching of phenanthrene fluorescence by anthracene in cyclohexane indicates both dynamic and static energy transfer mechanisms. For phenanthrene sorbed on mineral surfaces, the dynamic pathway is limited. Therefore, the appearance of anthracene fluorescence due to energy transfer from excited phenanthrene can serve as a spectral probe for polyaromatic hydrocarbon (PAH) aggregation and association on mineral surfaces.