This work aimed to use continuous measurements of viscoelastic properties to evaluate the effect of hydrothermal treatment on poplar samples. Different conditions (temperature and pre-soaking liquid: acidic, neutral and alkaline) were tested on wood in both tangential and radial directions. Two viscoelastic properties were determined: the modulus of elasticity and the stress relaxation. The applicability of these properties as indicators of the kinetics of biomass deconstruction was also evaluated, thanks to the chemical analyses performed on the treated solid and the recovered liquid phase. The ultimate goal is to build a macroscopic indicator capable of establishing rules to optimize the hydrothermal treatment before the explosion stage. The joint use of the two parameters succeeded in revealing the effects of chemical degradation, including the coexistence of cleavage and re-condensation and the impact of process conditions (temperature, residence time, and pre-soaking liquid). The monotonous behavior of stress relaxation is a major asset as a possible macroscopic indicator of biomass deconstruction.