Penicillium simplicissimum, a coal-degrading fungus, was shown to utilize aromatic coal-substructure compounds as the sole carbon source. Aromatic compounds were also mixed with soluble coal polymer and solubilization was studied over a 7-day period. The degradation of coal in supplemented samples measured as the amount of acid-precipitable coal polymer being formed was investigated. Although coal solubilization was produced with most of the aromatic compounds tested, wide differences in the extent of solubilization in the presence of each compound were recorded, depending on whether the intact organism or cell-free extract was used. The efficient conversion of the compounds by the fungus is related to the efficiency shown in the degradation of coal. The infrared spectra and elemental analysis of biodegraded coal and the undegraded parent material were used to draw conclusions concerning the cleavage of functional groups in the coal. Possible involvement of enzymes in the process is discussed.