Abstract Autohydrolysis of wood as a pretreatment stage before alkaline pulping is seen as an opportunity to approach the ideals of a forest biorefinery. In autohydrolysis the degradation products of primarily hemicelluloses and some other wood components are mainly dissolved (liquid phase) and they also escape partly as gas. In the present paper, birch was autohydrolysed at two intensity levels (characterised by P-factors of 200 and 1000) and the mass balances of autohydrolysis including solid, liquid, and gaseous phases were established. Approximately 10% and 25% of the total wood substance was transferred to the liquid phase at the two autohydrolysis intensities, respectively. In both cases about 50% of the degraded wood substance consisted of xylose-based material. The share of monomeric xylose was about 6% and 50% in the two samples, respectively. From the residual oligosaccharide fractions, 13% and 90% accounted for the DP2 to DP4 components in the P200 and P1000 samples, respectively. As expected, the weight-average molar mass of the components in the P1000 hydrolysate was substantially lower (0.4 kDa) than those in the corresponding P200 hydrolysate (2.1 kDa). Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) measurements revealed that relative to xylotriose the P1000 sample contained significantly fewer acidic pentosan fractions than the P200 sample. Xylooligosaccharides dissolved at both intensity levels remained highly acetylated, and some of these also carried 4-O-methylglucopyranosyluronic acid substituents.