In this paper, the viscoelastic properties of wood under water-saturated conditions are investigated from 10 to 135 degrees C using the WAVE(T) apparatus. Experiments were performed via harmonic tests at two frequencies (0.1 and 1 Hz) for several hours. Four species of wood were tested in the radial and tangential material directions: oak (Quercus sessiliflora), beech (Fagus sylvatica), spruce (Picea abies) and fir (Abies pectinata). When the treatment is applied for several hours, a reduction of the wood rigidity is significant from temperature values as low as 80-90 degrees C and increases rapidly with the temperature level. The storage modulus of oak wood is divided by a factor two after 3 h of exposure at 135 degrees C. This marked reduction in rigidity is attributed to the hydrolysis of hemicelluloses. The softening temperature of wood is also noticeably affected by hygro-thermal treatment. After three short successive treatments up to 135 degrees C, the softening temperature of oak shifted from 79 degrees C to 103 degrees C, at a frequency of 1 Hz. This reduction in mobility of wood polymers is consistent with the condensation of lignins observed by many authors at this temperature level. In the same conditions, fir exhibited a softening temperature decrease of about 4 degrees C. In any case, the internal friction clearly rises.