A study of the shrink-age of cubic (similar to5 mm) birch wood particles during pyrolysis is presented. The particles were rapidly injected into a preheated furnace with a constant temperature in the range 350-900degreesC. The size of the particles in longitudinal, tangential and radial directions was measured until no further mass loss occurs. The volume shrink-age was found to be 45-70% and the shrinkage in the different directions 5-25, 25-40 and 15-40% for longitudinal, tangential and radial directions, respectively. Longitudinal shrink-age commenced after about 60% mass loss and is not strongly dependent on heating rate or on cellulose chain scission. A maximum shrinkage was found for tangential and radial directions at 400 and 500-700degreesC, respectively, and above these: temperatures the shrinkage decreases. The char yield decreases and the char structure becomes more deranged with increasing temperature. Empirical models of shrinkage as a function of conversion are presented.