A number of geological flint pieces collected at several sites in northwest Greece were examined using electron spin resonance (ESR) spectroscopy. The aim was to determine characteristic peaks which could be associated with the geology of a particular region. In addition, a data basis was created for the behavior of the ESR peaks when the flints were heated in the laboratory. Most of the peaks were identified and found to originate from oxygen associated trapped holes, radiation-induced trapped electrons, and organic radicals including CH₂ and CH₃. Although the variation of the peaks in the unworked geological flint is fairly random, there are combinations which could characterize a particular area. The changes in the radiation-induced peaks and the organic radicals with heating which make possible the identification of archaeological flints heated in Palaeolithic times appear very promising. The firing temperature of a number of archaeological flint artifacts was determined with very good accuracy.