The effect of the height of the bottom bed on the conditions in the lower part of a 12 MW CFB boiler was studied. Pressure measurements along the furnace height were made together with gas (CO, hydrocarbons, O-2, NO, and SO2) and zirconia-cell probe measurements in the splash zone. The operation in the exploding bubble regime causes the particulate phase to be under reducing conditions. However, as the height of the bottom is decreased the conditions become more oxidizing. Additionally, the concentrations of NO and SO2 in the splash zone are significantly affected by the height of the bottom bed. Thus, the gas flow phenomena in the bottom bed are of great importance for the understanding of processes related to combustion, for instance formation and reduction of pollutants.