Unstable chemical species, that is, radicals generated by the thermal treatment of a dimethyl sulfoxide (DMSO) solution of the lignin of a softwood, Yezo spruce (Picea jezoensis Carr.), were studied in detail with an electron spin resonance (ESR) method combined with a spin-trapping technique. An unstable secondary carbon radical (similar toCH (.)) in the solution was trapped as a stable nitroxide spin adduct [R-(N-O (.))-CHsimilar to (R = tert-butyl benzene)] when the DMSO solution was heat-treated in the presence of a spin-trapping reagent [2,4,6-tri-tert-butylnitrosobenzene (BNB)] at about 40degreesC. This meant that alkyl phenyl ether bonds (similar toCH-O-phenyl), known as interunitary lignin bonds, were homolytically scissioned by the thermal treatment in the lignin solution. A detailed analysis of the ESR spectrum revealed that three kinds of radicals-primary (similar toCH(2) (.)), secondary (similar toCH (.)), and tertiary (similar toC (.)) carbon radicals-were trapped as stable spin adducts at about 60degreesC, although the phenoxy radical (Ph-O (.)) was not trapped by the BNB spin trap as the counter radical of the secondary carbon radical. This suggested that a fairly large steric hindrance existed between the so-called guaiacoxy radical with a methoxy group in the ortho position and the BNB molecule bearing two butyl groups as bulky moieties in the ortho positions. However, the phenoxy radicals in the lignin solution were stable up to about 60degreesC. (C) 2004 Wiley Periodicals, Inc.