This study investigates the chemical composition and bioavailability of thermally altered Pinus resinosa (Red pine) wood, focusing on the changes in chemical composition and the biological inertness of charred material. The wood was heated at temperatures ranging from 70 to 350 °C, and the resulting char was characterized using elemental analysis, solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). The heating process induced significant changes in the chemical composition, including dehydration and the formation of unsaturated structures. The NMR and DRIFT data indicated that the heating process converted O-alkyl carbon to aryl and O-aryl furan-like structures. These chemical changes significantly reduced the bioavailability of carbon in the charred samples, with the C mineralization rate constants decreasing by an order of magnitude for wood heated to ≥200 °C. The study also quantified the bioavailability of the thermally altered wood relative to unaltered wood, cellulose, and glucose, providing insights into the relationship between chemical structure and biological availability.This study investigates the chemical composition and bioavailability of thermally altered Pinus resinosa (Red pine) wood, focusing on the changes in chemical composition and the biological inertness of charred material. The wood was heated at temperatures ranging from 70 to 350 °C, and the resulting char was characterized using elemental analysis, solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). The heating process induced significant changes in the chemical composition, including dehydration and the formation of unsaturated structures. The NMR and DRIFT data indicated that the heating process converted O-alkyl carbon to aryl and O-aryl furan-like structures. These chemical changes significantly reduced the bioavailability of carbon in the charred samples, with the C mineralization rate constants decreasing by an order of magnitude for wood heated to ≥200 °C. The study also quantified the bioavailability of the thermally altered wood relative to unaltered wood, cellulose, and glucose, providing insights into the relationship between chemical structure and biological availability.