Heat generated persistent free radicals have been detected by electron spin resonance measurement of thermally modified wood, prepared by high pressure superheated steam treatment. The specific density of these radicals have been correlated with the durability against basidiomycetes, as measured by the European standard EN113 test. In the lower range of specific radical densities, the found correlation has revealed non-specificity for six different wood species, for two different treatment intensities and within different tree zones. This result promises a great potential for electron spin resonance to estimate the durability of high pressure steam modified wood. It also provides evidence for an antioxidant mechanism, responsible for the durability enhancement in heat treated wood.

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