In the course of almost 40 years various researchers, at what used to be TNO's Forest Products Research Institute, currently the TNO Centre for Timber Research, conducted studies into the physical properties of wood. The first studies and calculations were carried out by Mr E. Prochaska, after which Mrs G.M.C. Koning-Vrolijk continued the work. Indeed Mrs Koning-Vrolijk wrote the Institute's first publication (1962), an Eng lish version of which was published in 1963 (3) on the occasion of FAO and IUFRO Conferences held in the United States. Thereafter, the Institute's work was carried on by Mr A. Govers, Mr J.F. Rijsdijk and Mr P.B. Laming. Their research resulted in a second publication (Laming 1978) in which not only the mechanical properties but also the physical properties of 48 wood species were described. During the bulk of this period technical support was provided by Mr J.C. Verwijs and more recently by Mr L. van Brussel. After extensive studies, the Belgian Timber Information Institute also adopted the same research methods as TNO in order to obtain physical data on a number of wood species which were of commercial interest to the Belgian market but which had not been covered in TNO's studies. The Belgian Timber Information Institute's suggestion to include their research results, on a total of 17 wood species in this publication, .was therefore gratefully accepted.