Circulating fluidized bed (CFB) cyclones operate at high solids loadings. The paper presents, for the first time. particle trajectories within a cyclone obtained by positron emission particle tracking (PEPT), as a function of the solids loading (C-s). The pressure drop across a cyclone is a strong function of the solids loading. The objective of this work was to explain this behavior by direct observation of the particle movement. Cyclones normally operate in a stable particle movement mode, always with a spiral motion in the cylindrical part of the cyclone, followed by either a continued spiral in the cone (at low Q or by a much denser solids flow near the cone wall at higher C-s values. Data are used to obtain the tangential and axial velocity components of a tracer particle, the residence time of the particles in the cyclone, the thickness of the boundary layer in the cylindrical section of the cyclone, and the thickness of the dense wall layer in the conical section. This downward moving layer reduces the effective "free" cross section of the cyclone, thus increasing the air velocity and the pressure drop (Delta P), especially for small cyclones, as in the present research. This effect will be neolioible for larger cyclones. where the influence of solids film thickness is less important, when compared to cyclone diameter, and Delta P values are expected to remain almost constant with increases in the solids loading.