The study investigates the chemical composition and bioavailability of thermally altered Pinus resinosa (Red pine) wood. Char, formed by incomplete combustion, contributes significantly to soil organic carbon. However, its biological inertness depends on the extent of thermal treatment. The research quantifies how thermal treatment affects the chemical structure and bioavailability of char C from Red pine sapwood heated to temperatures between 70 and 350 °C. Chemical composition was analyzed using elemental analysis, solid-state 13C NMR, and DRIFT. Bioavailability was assessed through a 120-day laboratory incubation. Heating at temperatures ≥200 °C significantly altered the chemical composition, leading to dehydration and formation of unsaturated structures. NMR and DRIFT data showed conversion of O-alkyl C to aryl and O-aryl furan-like structures, consistent with thermal treatment of cellulose. These changes reduced microbial mineralisation of carbon in charred samples. C mineralisation rates decreased by an order of magnitude for wood heated to ≥200 °C. The study highlights that thermal alteration increases the proportion of aryl C, which is more resistant to microbial degradation. Char C is generally considered biologically inert, but its recalcitrance may depend on the extent of thermal alteration. The results suggest that char C formed at higher temperatures is less bioavailable, with a higher proportion of aromatic structures. The study also shows that the chemical changes in char are consistent with previous findings on cellulose and kerogen. The findings have implications for understanding the long-term carbon storage in soils and the role of thermal alteration in determining the bioavailability of organic matter.The study investigates the chemical composition and bioavailability of thermally altered Pinus resinosa (Red pine) wood. Char, formed by incomplete combustion, contributes significantly to soil organic carbon. However, its biological inertness depends on the extent of thermal treatment. The research quantifies how thermal treatment affects the chemical structure and bioavailability of char C from Red pine sapwood heated to temperatures between 70 and 350 °C. Chemical composition was analyzed using elemental analysis, solid-state 13C NMR, and DRIFT. Bioavailability was assessed through a 120-day laboratory incubation. Heating at temperatures ≥200 °C significantly altered the chemical composition, leading to dehydration and formation of unsaturated structures. NMR and DRIFT data showed conversion of O-alkyl C to aryl and O-aryl furan-like structures, consistent with thermal treatment of cellulose. These changes reduced microbial mineralisation of carbon in charred samples. C mineralisation rates decreased by an order of magnitude for wood heated to ≥200 °C. The study highlights that thermal alteration increases the proportion of aryl C, which is more resistant to microbial degradation. Char C is generally considered biologically inert, but its recalcitrance may depend on the extent of thermal alteration. The results suggest that char C formed at higher temperatures is less bioavailable, with a higher proportion of aromatic structures. The study also shows that the chemical changes in char are consistent with previous findings on cellulose and kerogen. The findings have implications for understanding the long-term carbon storage in soils and the role of thermal alteration in determining the bioavailability of organic matter.