Elephant grass (Pennicetum purpureum) and sugar cane bagasse slow pyrolysis experiments was carried out in a fixed bed reactor. A 20-cm internal diameter and 12-cm-long reactor was used. Particulate biomass filled up the reactor volume. Biomass was loaded into the reactor and heated in the axial direction using an electrical resistance located at the reactor's bottom. In order to control the temperature variation during the biomass pyrolysis process, four thermocouples were installed inside of the reactor. The remain residual mass was constant approximately after 73 min of heating; the running was stopped and remain carbonised; material was manually removed from the reactor. The residue formed three layer of biomass visually different described in detail here. Proximate analysis and higher heating value (HHV) tests were carried out to the material in each layer. Mass loss against time was recorded during experiments. The results indicated that the carbonisation ratio decreases in time because the carbon layer has low thermal conductivity and it does not permit proper heat transfer to the upper layer of biomass. It means that technology that avoids high-temperature gradients during the pyrolysis of bulk-dispersed biomass could avoid the problems described before. (C) 2004 Elsevier B.V. All rights reserved.