In the woodworking industry image analysis is routinely used for quality control and for matching and classification during various processes. An extension of these automated systems for the prediction of physical properties of thermally-modified wood (TMW) is enticing, because to date there is no a generalized procedure for the quality assurance of this material. The purpose of this work was to investigate the relationship between property changes in TMW and heated-induced colour changes. A description of the effect of the temperature and time of treatment on colour variables and its relationship between these and colour changes in Klason lignin is also given.