In new biorefinery processes, NMMO / water is used to pretreat biomass in order to increase the efficiency of subsequent digestion processes, while GVL / water is used for “Organosolv” fractionation of biomass. The combination of both methods, GVL digestion after preactivation by NMMO, appears useful, but was not successful. In the present study we investigate the cause of this failure and examine the chemical processes in the ternary system NMMO / GVL / water and in the quaternary system NMMO / GVL / water / “biomass”. The consumption kinetics of NMMO and GVL at different temperatures, water contents and NMMO / GVL ratios were recorded. The corresponding degradation and reaction products were identified for the first time by combining nuclear magnetic resonance (NMR) spectroscopy and gas chromatography-mass spectrometry (GC-MS) techniques and the synthesis of authentic compounds for comparison. Degradation products of NMMO and GVL alone as well as reaction products of both components together (α-morpholinomethyl-GVL (7th), α-methylene GVL (8th) and 4-hydroxyvaleric acid morpholide (13)) were observed among the main breakdown products. At temperatures of 150 ° C and a water content <10% (or 180 ° C or 30%), the NMMO contained was autocatalytically degraded in strongly exothermic reactions and explosive processes occurred which resulted in complete carbonization of the reaction mixture. The system remained stable at higher water contents, but NMMO was still completely degraded and GVL was consumed to a considerable extent. While the biomass component cellulose was largely unreactive, lignin was the main responsible for the degradation reactions in the system. The formation of NMM (5) from NMMO and the resulting ring opening of GVL to 4-hydroxyvaleric acid (3), which is immediately oxidized to levulinic acid by NMMO (4th) were the first reactions that triggered the subsequent, more complex degradation pathways. The chemical structures of all degradation products were completely analytically confirmed. Due to the instability of the NMMO / GVL system, the combination of NMMO biomass pretreatment and GVL biomass digestion is prohibited unless the NMMO has been carefully removed beforehand. In addition to these practical conclusions with regard to biomass processing in biorefineries, the present study will hopefully provide helpful insights into the chemistry of NMMO and GVL and the underlying reaction mechanisms.