To clarify the mechanism of irreversible dimensional changes due to the hygrothermal treatment of green wood, i.e., the hygrothermal recovery (HTR), the changes in vibrational properties of compression wood and normal wood were measured after hygrothermal treatment at 60 degrees C, 80 degrees C, and 100 degrees C. In addition, the relationship between those changes and HTR strains were discussed. The hygrothermal treatment induced an increase in mechanical loss tangent (tan) and decrease in specific dynamic Young's modulus (E/). It seems that the changes in vibrational properties due to hygrothermal treatment had a time-temperature dependency: Higher temperatures and longer treatment durations induce larger increases in tan and larger decreases in E/. In contrast to the quenching effect, tan and E/ did not recover to their original state even after 60days of conditioning in water at 20 degrees C. For compression wood with a large microfibril angle (MFA), there was a clear relationship between the changes in vibrational properties and HTR strains. The tan increased and E/ decreased with hygrothermal treatment, corresponding to dimensional changes in the L-direction. This suggests that structural changes in wood components are responsible for HTR. The most likely mechanism for HTR is that the hygrothermal treatment softens the lignin to release locked-in growth stress. Subsequently, irreversible structural changes in lignin induce both the changes in vibrational properties and HTR. For normal wood, because of the small MFA, the structural changes in lignin in the L-direction are possibly restricted by crystalline cellulose. As a result, the relationship between the changes in vibrational properties and HTR is uncertain.