The development of new highly efficient combined-cycle power plants is fundamentally linked to the development of new hot flue gas purification systems. Hot flue gas purification systems aim to remove particles and newly formed particulate matter from condensation reactions from the flue gas to prevent any damage to the gas turbine. This study focuses on the characterization of particulate matter which was deposited onto the inner walls of flue gas probes in a 1 MW pilot plant for pressurized pulverized coal combustion under slagging conditions (PPCC). The particulate matter in the flue gas stream originates from incompletely separated fly ash particles and particles formed during condensation processes from the hot; flue gas. The phase analysis of the materials was done by high-resolution scanning electron microscopy. The combination of wavelength dispersive electron microprobe with a knowledge of the approximate temperature profile through the flue gas probe allowed the definition of dew points of single crystalline phases under the actual pressure conditions from the hot flue gas in the pilot plant. At high temperatures ( >1200 degrees C) the deposits on the inner wall of the flue gas probe are mainly fly ash particles which were deposited in the liquid state. With decreasing temperature, CaSO4, Na2SO4, K2Ca2(SO4)(3), and K3Na(SO4)(2) condense from the flue gas. The ubiquitous occurrence of crystalline Cr2O3 can be traced to high-temperature refractory materials in the boiler and the subsequent particle separator of the pilot plant. At low temperatures some sulfates may decompose and form highly corrosive pyrosulfate melts. The changes in the phase composition of the deposits correlate well with chemical profiles along the flue gas probe.