Petroleum coke firing in a circulating fluidized bed boiler has sometimes been reported to be associated with cyclone deposit problems and return leg plugging. In this paper we present data which indicate that some of these problems may be due to the calcium-rich bed material used during the firing. We have earlier shown that calcium oxide may react with the flue gas components SO2 or CO2, causing neck growth between the solid particles. This neck growth between particles may lead to both deposits in the cyclone and plugging of the cyclone return leg. In this study we made use of a sintering testing method, based on compressive strength tests of heat-treated cylindrical pellets. Laboratory-prepared petroleum coke ash was mixed with two potential bed materials, limestone and dolomite, and sintering tests were performed in three different gas atmospheres. Significant differences were found between the mixtures as a function of both the gas atmosphere and temperature. We also performed thermogravimetric analyses on one of the bed materials, the limestone. Based on these results a mechanism for the formation of cyclone deposits and bed material agglomeration in the return leg was suggested.