Mild thermal treatments (below 160°C) are increasingly considered as a mean to accelerate some beneficial effects of wood ageing. In this work, treatments were done in a pressurized reactor which has controlled conditions of temperature and relative humidity. We performed 5 treatments, each one applied to poplar (for study of dimensional stability in view of panel paintings applications) and to spruce (for study of axial and radial acoustic properties in view of musical applications). Three groups were treated at 150°C with, respectively, 0% (constant) relative humidity (RH), 25% constant RH, and cyclic variations between these 2 RH. The same protocol was applied to three other groups at a temperature of 130°C. The oven-dry weight, dimensions and colorimetric properties and the air-dry (3 weeks at 20°C and 65% RH) weight, dimensions and viscoelastic vibrational properties were measured before and after treatment. Intermediate data were also measured in the course of the treatments for weight, dimensions and colorimetric properties, in order to model the kinetics of modification. Although oven-dry weight loss were moderate (maximum 4%) in these relatively mild conditions, properties were significantly improved. Moisture sorption and swelling/shrinkage were significantly reduced, as well as vibration damping, without negative effects of the rigidity. The effect of reconditioning at high relative humidity was also investigated.