The origin of pressure fluctuations in an internal-loop airlift reactor (ILALR) and its application in the flow transition detection are investigated. It is found that pressure fluctuations can be characterized as global pressure fluctuations and local pressure fluctuations by frequency domain analysis and wavelet analysis. The global pressure fluctuations generated by gas compression in the gas plenum and flow fluctuations in the gas-supply system have almost a linear attenuation in the downcomer and almost no attenuation in the riser, especially in heterogeneous flow regime. However, it is found that the pressure wave from bubble eruption at bed surface has little impact on the wall pressure fluctuations. The global pressure fluctuations may be explained by Sasic's model. The local bubble-induced pressure fluctuations generated by bubble passage, coalescence and breakage can be determined by bubble passage frequency bandwidth and lower coherence. After extracting the local bubble-induced pressure fluctuations from the origin wall pressure fluctuations, it is shown that the Hurst exponent of the local pressure fluctuations increases faster in the homogeneous flow regime than in the heterogeneous flow regime, which can be employed to indicate the flow regime transition. (C) 2008 Published by Elsevier Ltd.