Combined straw-bitumen pellets have been proposed as an alternative fuel. An interesting finding is the potentiality of straw ash constituents to retain sulphur as bitumen that has relatively high sulphur content. The aim of the present work is to enhance sulphur self-retention to directly meet the environmental regulations by building-in CaO in the pellet instead of feeding sorbent separately. CaO powder has been mixed with the pellet constituents during production processes. An experimental study has been carried out to investigate the effectiveness of built-in CaO to retain sulphur. Two experimental facilities have been utilized. A fluidized bed combustor of 100 mm ID is used to perform batch tests, whereas a pre-pilot scale combustor of 300 mm ID is used for continuous operation. The obtained results demonstrate that building-in CaO is a very efficient technique for retaining sulphur. A major portion of sulphur is retained within the pellet throughout the entire burning period, in particular, during char burning stage. After being a completely burned pellet the remaining ashes work as a very efficient sorbent within the combustor. Typically, at built-in Ca/S=0.75, SO2 is reduced to about 65 ppm corresponding to 96% sulphur retention efficiency. (C) 2006 Elsevier B.V. All rights reserved.