Swelling and shrinking of solid wood and wood composites are deeply investigated for a long period. Swelling and shrinking of a relatively novel type of material, wood-plastic composites (WPC) have not yet been investigated in depth. In contrast to classical wood composites, WPC are compounded and extruded or injection moulded at high temperatures of up to 200 °C and at high pressures of up to 2000 bar. Moisture levels of freshly produced WPC are below 1 %. In this study comparison of compression at uniaxial load, relaxation, swelling and swelling pressure behaviour of solid wood, wood powder and WPC has been done. Part 1 starts with compression at high temperature and high pressure of solid wood, wood powders and WPC. Part 2 will be about relaxation, swelling and swelling pressure. Solid wood cubes build up high swelling force when restrained, and show a high volume swelling if allowed to swell unre-strained. WPC shows a comparable high swelling force under restrained condition, but only one tenth of the volume swelling of solid wood and wood powders.