Hydropyrolysis (hypy) as a fast means for preparing lignin concentrates for stable carbon isotope analysis was evaluated by performing hypy at different temperatures (200–500 °C) on Klason lignin concentrates, cellulose, and wood samples followed by bulk stable carbon isotope analysis of the solid residues. The δ13C values of cellulose, Klason lignin and their hypy residues revealed that hypy does not produce isotopic fractionation throughout the temperature range explored. The δ13C of hypy residues from cellulose-Klason lignin physical mixtures obtained at 500 °C were in the value range of the corresponding Klason lignin indicating that cellulose decomposed completely and that no major structural rearrangements, which could lead to carbon cross-linking between these two moieties are produced by hypy. The hypy residues obtained from wood samples at 500 °C were consistently depleted in 13C by 0.8‰ and enriched by 1.6‰ relative to the original wood and Klason lignin, respectively. This finding results from polysaccharide derivatives remaining in the wood residue, with a percentage of polysaccharide derived carbon 1.9 and 3.7 times higher than expected at 300 and 350 °C, respectively. It is concluded that the results presented here provide an indicator of the extent of cross-linking between lignin and polysaccharides (hemicellulose) in the native wood forming lignin–carbohydrate complexes.