Heat treatment at relatively high temperatures (ranging from 150 degrees C to 260 degrees C appear to be an effective method to improve the durability of wood. This study investigated the reasons for the decay resistance of heat-treated and untreated wood as composed of polymeric structural constituents by solid-state CP-MAS C-13-NMR analysis after fungal exposure in ground contact. An industrially used two-stage heat treatment method under relatively mild conditions (< 200 degrees C) was used to treat the samples. Fungal exposure in ground contact resulted in strong degradation of the carbohydrates (cellulose and hemicellulose) of treated and untreated Scots pine, Radiata pine, and Simaruba. Fungal attack of the carbohydrates appeared to occur mainly at C4, resulting in cleavage and eventually depolymerization of cellulose and hemicellulose. The CPMAS C-13-NMR spectra of the heat-treated wood revealed similarities but also clear differences after fungal exposure in ground contact with the untreated wood. In ground contact fungi appeared to attack the carbohydrates of heat-treated wood at C1 and possibly at C4 in order to cleave and eventually depolymerize cellulose and hemicellulose. An attack on the out-of-the-ring alcoholic group, -CH2OH, of the carbohydrates of the heat-treated wood was observed (particularly in treated Radiata pine). The fungus possibly tried to cleave the out-of-the-ring CH2-OH group on the main H-bond fixing sites of the crystalline cellulose structure in order to open the cellulose crystalline structure to an amorphous structure to decrease its water repellency and facilitate enzymatic cellulose degradation; this was also observed, but to a lesser extent, in untreated Radiata pine and untreated Scots pine. The opening of the glucose pyranose ring in heat-treated Simaruba after fungal exposure, not observed in the untreated wood, was remarkable, and the thermal degradation of alpha-arabinofuranose during heat treatment indicated more extensive decay. Demethoxylation and ring opening of the aromatic structure of lignin were observed, especially in the heat-treated Radiata pine, Douglas fir, and Simaruba. (c) 2006 Wiley Periodicals, Inc.