The dynamic behaviour of gas-solids flow in a CFB riser is characterized by Fourier spectral analysis of pressure fluctuations. Time series of pressure fluctuations were recorded at different measurement positions in a lab-scale riser at gas velocities of 3.0, 4.0, and 5.0 m/s under dilute and dense flow conditions, with solids mass fluxes ranging from 0 to 232 kg/m(2)s. When dense flow conditions are met at the measurement position, two regions with different power-law fall-off are observed in the power spectral density. At dilute flow conditions, no clear regions with a power-law fall-off are observed. The power in region I shows a sharp decrease from a value of approximate to 2- approximate to 0.5 at the transition from dilute to dense flow at the measurement position. With increasing mass flux, the power increases to a value of approximately 2 again. The power in region II increases from a value of approximate to 3.5- approximate to 5 for all measurement positions when a dense bed is formed in the bottom section of the riser. With the coherence between measurement positions, it is demonstrated that gas/particle turbulence is a negligible source for pressure fluctuations. Gas velocity fluctuations and solids clusters are the predominant sources for these fluctuations. The power-law fall-off regions are attributed to solids clusters, and explained by the typical triangular or saw-tooth shape of the pressure fluctuations.