2,2-Diphenyl-1-picrylhydrazyl (DPPH) is reversibly adsorbed at acidic centers on the surfaces of a variety of common mineral pigments and fillers, and undergoes a subsequent irreversible reaction with adsorbed water or surface hydroxy groups to yield 1,1-diphenyl-2-picrylhydrazine, 1-(4′-nitrophenyl)-1-phenyl-2-picrylhydrazine, and 4,4′-oxybis[N-(4-picryliminocyclohexa-2,5-dienylidene)aniline] as the major products. The rate of the irreversible reaction on kaolinite is second-order with respect to DPPH and probably depends on the disproportionation between a DPPH and a protonated DPPH molecule. The possible use of DPPH as a probe for the activity of mineral fillers in polymer chemistry, and some implications on its use as a diagnostic test for radical species are discussed.