Abstract Microcrystalline cellulose was charred anaerobically at atmospheric pressure and at different temperatures ranging from 190 to 390°C. Temperature-resolved, in-source-filament pyrolysis, ammonia chemical ionisation mass spectrometry (Py-CIMS) of these chars shows ion series of ammonium adduct ions of anhydro-oligosaccharides. These anhydro-oligosaccharides were identified by high-performance liquid chromatography (HPLC) of derivatised, off-line, Curie-point-pyrolysis condensates as β-(1 → 4)-linked glucose oligosaccharides with an 1,6-anhydrosugar terminal at the reducing end. These anhydroglucooligosaccharides were observed in chars even after 2.5 h heating at 270°C. The relative amount of preserved glucose in the chars was determined by acid hydrolysis. Coexistence of two solid phases: (i) residual crystalline cellulose and (ii) a condensed thermostable phase was observed in chars derived from cellulose by heating at 250, 270 and 290°C. It is believed that the formation of this new condensed phase is initiated in the thermally induced, amorphous domains of cellulose. No ether-bonded products of inter- and intra-chain crosslinking reactions were detected in the acid hydrolysates of the chars.