Detailed microscopic examinations have been conducted on two, temperature-regulated probes which had been exposed to biomass flue gas inside a PF boiler for 3770 h at 600 degrees and 650 degreesC respectively. The two probes were made from commercial grade heat exchanger alloy (TP 347H FG). A significant buildup of potassium chloride and sulphate was found on both probes within the deposits that sinter more strongly on the 650 degreesC probe. Both probes suffer extensive intergranular corrosion as well as general corrosion. The higher temperature enhances scale formation which may provide a beneficial effect with respect to hot corrosion resistance. The intergranular corrosion is linked to the presence of sulphur. The overall material wastage is almost the same for both probes and for different locations covered by deposit.