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Abstract: Some fossil wooden samples of the Tertiary period, extracted from the crater of the kimberlite BHP-Panda pipe from the Lac de Gras region (in the Canadian sub-Arctic area), and previously identified as Sequioxylon canadense Blokhina, were chemically examined to both assess their state of preservation and discuss about the possible causes of their decay. The chemical composition of fossils (in terms of holocellulose, lignin and ash amounts) was evaluated by means of wet analyses, together with the chromatographic analysis of dichloromethane extracts, and additionally measurements of the cation exchange capacity (CEC) of fossils were also carried out. The analyses revealed the complete loss of holocellulose (except for 2 out of 9 samples), whereas lignin became the predominant component, together with ash (whose amount in one case was as high as 47.5%). Moreover, CEC data showed that all acid carboxyls were transformed into carboxylates. They represented the most important form of residual carboxyl groups in the fossils, and were even more abundant than in the reference (fresh) wood. All these occurrences evidenced a generally high level of decay for the fossils, as a consequence of water leaching during burial, which determined both the hydrolysis of esters groups and the partial depolymerisation of lignin. [Copyright &y& Elsevier]