Heat-induced weight loss (WL) and chemical and dimensional changes of small specimens of beech ( Fagus sylvatica L.), Scots pine ( Pinus sylvestris L.) and Norway spruce ( Picea abies L.) wood were examined after thermal modification in the 190–245 °C temperature range. Treated specimens exhibited reductions in their oven-dry weight in line with the severity of the treatment, with the effect of increasing the temperature of exposure being greater than extending the period of treatment. Wood polysaccharides were found to be distinctly more labile than the lignin constituent; the latter increased possibly as a result of repolymerisation reactions trapping some degradation products in the process. Specimens shrank in the transversal plane in a tangential to radial ratio of 2:1 regardless of the treatment regime, while their length increased marginally for WL < 10–12%. It is proposed that the thermal modification leaves the cell wall material in a permanent strained state.