The extent of softening during biomass pyrolysis is of high importance for fundamentals and reactors design but was not yet quantified in situ. We provide the first in situ rheology and H-1 NMR analysis during the pyrolysis of biomass (Miscanthus), microgranular cellulose, ethanol organosolv lignin, and xylan. In situ rheology reveals the viscoelastic behavior of the materials. The softening, resolidification (char formation), swelling, and shrinking behaviors are quantified during pyrolysis in real-time. These phenomena are discussed. The H-1 NMR analysis gives the mobility of protons developed during pyrolysis. A viscous and mobile material is formed during cellulose and xylan pyrolysis, even at a slow heating rate (5 K min(-1)), by products at liquid phase under reaction temperatures. For lignin, a soft material and mobility are first formed by glass transition phase, then overlapped with covalent bond scission, leading to a completely mobile material. The comparison between H-1 NMR and rheology results shows that mobile liquid-like products are trapped inside elastic solid-like cavities. Strong interactions between polymers in the native biomass network are evidenced. Cellulose tends to impose its visco-elastic behaviors to the polymers network during biomass pyrolysis. Rheological signatures are also of practical interest to design feeders and reactors for polymers biosourcing, biomass gasification, or combustion.