Wood-water relationship of untreated and heat-treated wood was studied. Specimens of Eucalyptusgrandis, E. saligna, and E. citriodora were submitted to five conditions of heat treatment: 180°C and 220°C with air; 220°C, 250°C, and 280°C with N2. The wood-water relationships were accurately studied in a special device, in which the moisture content (MC) of the sample was measured with a highly sensitive electronic microbalance placed in a climatic chamber. The dimensions of the sample were collected continuously without contact by means of two high-speed laser scan micrometers. Sorption curves and shrinkage-MC relationships were observed. To study the effects of heat treatment, the following parameters were also determined: fiber saturation point (FPS), wood anisotropy (T/R ratio), shrinkage slope, reduction in hygroscopicity, and anti-shrink efficiency (ASE). The physical properties were significantly affected only at 220°C and above. At heat temperature levels higher than 220°C, the reduction in hygroscopicity and ASE are higher than 40% and continue to be reduced with increasing temperature level. This work also demonstrates that heat treatment does not change the slope of the curves “shrinkage vs. MC”, proving that heat treatment affects the domain of alterations in wood properties, but not the behavior within this domain.