Agglomerated bed material and deposits from various surfaces in the bed area and in the flue gas duct of pressurized fluidized bed combustion (PFBC) plants were examined by chemical and X-ray diffraction methods. Crystalline compounds present in the deposits were identified and possible mechanisms leading to the formation of bed material agglomerates, deposits and fouling on surfaces in the flue gas duct are discussed. The results show that all of the chemical systems CaS-CaSO4-alkali species, silicate-alkali-chlorine and silicate systems containing iron-rich oxides are important in the sintering and fouling process.