For successful operation and design of a bubbling fluidized bed reactor handling a specific biomass, in-depth knowledge about the bed behaviour is paramount. This study compares the behaviour of a bed of sand containing wood pellets with that containing wood chips at different gas velocities and biomass proportions in a cold fluidized bed of diameter, 10.4 cm. The density and volume-equivalent spherical particle diameter of the pellets are 1139 kg/m(3) and 8.96 mm, respectively while those of the wood chips are 423 kg/m(3) and 6.87 mm, respectively. The results show that at low gas velocities, wood chips segregate upwards while the pellets segregate downwards in their respective beds. The spread of biomass towards the walls is higher in the bed with wood chips than in that with wood pellets. As the biomass load increases, the bubble diameter increases and the transition from bubbling to slugging regime gets smoother, resulting in an increase in the minimum slugging velocity. The minimum gas velocity for effective solids mixing is less dependent on the bed height, but increases with increase in the biomass load and decreases with increase in the bed diameter. However, when slugs flow in the bed, the biomass layer at the bed surface plugs, preventing mixing of particles to be achieved at the desired gas velocity. A mechanistic model is developed for predicting the minimum gas velocity required to achieve an effective mixing at the surface of a segregated bed. Although this study is conducted in a cold bed, this same model is considered important for a hot bed reactor since devolatilization enhances the upward flow of biomass due to reduction of the biomass density.