Between 0.5 and 1.1% mean maximum reflectance of vitrinite (Ro_max), the log mean fluorescence intensity of vitrinite is shown to exhibit a negative linear relationship with iso-rank variation of vitrinite reflectance. This relationship is used to devise a method to both identify reflectance-suppressed vitrinite and to estimate the extent of suppression. Iso-rank variation of vitrinite reflectance and fluorescence intensity observed in one Late Cretaceous and three Eocene age coal seams is attributed to the activity of methanogenic and sulfate-reducing bacteria.