A densitometric technique was developed that permitted rapid quantification of lichen compounds separated by thin-layer chromatography. Thin-layer plates were scanned in a densitometer using a UV light source and density-dependent absorbance or fluorescence was measured for each compound. The lichen products tested differed with respect to photostability under UV, and thus in the time taken to achieve constant densitometric readings. Minimal exposure times required for stabilization of the compounds tested ranged from 3 min to 2.5 hr.