Electron spin resonance (ESR) has been used to study chemical radical species in archaeological samples. Radicals become trapped in solids, and flint is a very receptive medium. Methyl radicals, formed from unknown precursors, are trapped in cavities. More recent work now has detected perinaphthenyl radicals, also probably trapped in cavities, in both British (Grimes Graves) flint and Middle Eastern chert. Archaeological heating of a flint sample can be detected by ESR. Only a small number of flint sources give the ESR signal of the radical so the information may assist provenance studies of flint. -- AATA