In art effort to clarify the causes of agglomeration and fouling in fluidized bed combustion of petroleum coke, a detailed study was made of samples taken from different locations of an industrial-sized CFBC bailer, including deposits formed after 7 and 96 days of operation. It was found that vanadium, the suspected cause of the agglomeration, does not accumulate in fouled regions and that no low melting oxides were present. Neither could any low melting eutectics be expected from the vanadium compounds identified Therefore, the high concentrations of vanadium in the petroleum coke fuel cannot explain the formation of agglomerates. Fouling is attributed to molecular cramming effect caused by the nearly quantitative conversion of the CaO to CaSO4 and the absence of fuel-derived ash providing inert material, which could contribute discontinuities between the sintered anhydrite grains and prevent massive consolidation of deposits.