The characteristics of the fluid-dynamic boundary layer (FBL) were studied in the 12 MW circulating fluidized bed boiler at Chalmers University of Technology. For a comparison, some additional measurements were made in a 165 MW CFB boiler. The FBL consists mainly of the falling film of particles at the furnace wall. The distance from the wall to the zero net particle flux position is used as a measure of the thickness of the layer. Horizontal profiles of particle flux, velocity and concentration were measured in the transport zone of the furnace. It was found that the FBL thickness varies only slightly or not at all with particle concentration, gas velocity and height in the furnace. An estimation of the thickness is given as a function of the horizontal bed dimension. The particle falling velocity inside the FBL is about 0.9-1.6 m/s depending on location. There is a sharp increase in particle concentration in the FBL all the way towards the wall. The disparity between the particle flux profile normal to a plain refractory-lined wall and the corresponding profile normal to a membrane-tube wall can be compensated for by taking the fin-tube configuration into account. From this point of view, the particle redistribution mechanism at the membrane-tube wall is discussed.