Two wood species - the softwood Pinus radiata and the hardwood Eucalyptus regnans have been examined by CP/MAS13C NMR spectroscopy before and after treatment by two steam explosion processes, namely, the Canadian Iotech Process and the Australian Siropulper process.1,2While the main effects of treatment appeared to be, in general, similar for both processes and both species of wood, e.g. an apparent increase in cellulose crystallinity, hydrolysis of the hemicellulose component, de-etherification and depolymerization of the lignin component, some differences were also detectable, particularly in samples of extracted exploded wood. These included differences in cellulose crystallinity, composition and residual lignin structure. In some cases, signal area ratios could be used both to confirm and to provide a comparative measure of these effects and differences. Signal ratios were also the basis for development of a useful, non-destructive method of examining lignin content, which may have an advantage over the Klason method for comparing exploded wood and extracted exploded wood samples, in that it does not degrade carbohydrate. Some implications of the results for bioconversion processes are mentioned briefly. CP/MAS NMR spectroscopy is thought to be a valuable and promising technique for examining and comparing the effects of pretreatment on the structure and composition of biomass substrates, as it provides estimates, from a single experiment, of cellulose crystallinity and lignin content, as well as qualitative comparisons, e.g. of lignin structure.