The purpose of the present work is to study the influence of the pressure drop across the air distributor on the bubbling conditions of the bottom bed of circulating fluidized beds (CFB). The bottom bed is the dense bubbling zone just above the distributor. The experimental work was carried out in a 12 MW(th) CFB boiler and in a cold CFB. Three different distributions of the bubble flow in time and space, termed fluidization regimes, were identified in the cold CFB : the multiple bubble regime with many small bubbles evenly distributed in the bed; the single bubble regime, characterized by the presence of only one bubble at a time in the bed; and the exploding bubble regime with large, single, irregular voids stretching from the air distributor to the bed surface. These bubbling conditions were observed during variations in the gas velocity and the distributor pressure drop. A comparison with the 2 m(2) cross-section CFB boiler showed that the boiler always operates in the single or in the exploding bubble regime, which indicates a bubble flow that is not continuous and not well distributed over the cross-section of the bed. The conditions in the boiler are influenced by the relatively large area of gas passage and the low pressure drop of the boiler air distributor.