When flints are examined using electron spin resonance spectroscopy, many show a thermally-sensitive maganese (II) signal when heated between 300°C and 400°C. Three distinct types of manganese (II) signal modification are described and the possible mechanisms for this thermally-induced behavior are discussed. Manganese (II) can be used to complement radical carbon as an indicator of ancient heat treatment in flint artifacts. See AATA 16-842.