In later years scientists have taken increased interest in understanding wood ultrastructure, but with limited emphasis on close molecular relationships of its polymers. This aspect is examined from the Hansen solubility parameter concept and from the requirement that contacts between wood polymers strive to reduce the free energy. Polymer segments with matching solubility parameters tend to locate near each other. Wood polymers have larger solubility parameters than the test solvents used to the test solvents used to characterize them, but the use of model compounds and group contribution methods yields consistent solubility parameters for the key wood components. Lignin and cellulose are not compatible. Cellulose regions and backbones of hemicelluloses are compatible to a degree. Some hemicellulose segments with hydroxylated side groups are expected to orient themselves towards cellulosic regions, whereas segments, e.g. with acetylated side groups, orient themselves towards lignin regions, in both cases by virtue of similarity of their local solubility parameters. The diverse nature of the side groups is expected to determine the movements of water and monomer species through regions of similar solubility parameters in the fibre wall. Hemicellulose molecules may possibly be contained within cellulose regions with the backbones and hydroxylated side groups towards the cellulose and the acetylated side groups inwardly. Hemicelluloses are generally likely to act as "surfactants", binding the high energy cellulose regions to the lower energy lignin regions. Though wood cannot be dissolved, some "strong" solvents may alter its ultrastructure, while others do not affect the structure in spite of extreme softening. Short introductory recapitulations are given of the general knowledge of ultrastructure and of the Hansen solubility parameter concept.