A new and original method for flow regime identification in multiphase reactors (bubble columns, fluidized bed and spouted beds) was developed. The approach is based on the information entropy (IE) theory and division of the signal (photon counts, differential or gauge pressure fluctuations) into different regions. On the basis of two well-pronounced drops in the maximum information entropies IEmax extracted from photon counts in a bubble column (0.162 m in ID) operated with an air-therminol LT system, two transition velocities U-trans (0.02 and 0.09 m s(-1)) were identified at ambient pressure P. They delineated the boundaries of bubbly flow, transition and churn-turbulent regimes. The existence of two U-trans, values was confirmed by the Kolmogorov entropy (KE) values. At P=0.4 MPa, the first U-trans shifted to somewhat higher value (0.04 m s(-1)), whereas the second U-trans was identified earlier (at 0.07 m s(-1)). At P=1.0 MPa both first and second U-trans values shifted to higher values (0.05 and 0.1 m s(-1)). It was found that in an air-water bubble column (0.1 m in ID) the IEmax values (extracted from photon counts) were capable of identifying only one U-trans value (0.02 m s(-1)). In a fluidized bed (0.44 m in ID) operated with an air-polyethylene system, the minimum fluidization velocity U-mf was identified at 0.103 m s(-1) based on the well-pronounced drop in the IEmax values (extracted from photon counts). The IE theory was also applied to differential pressure fluctuations measured in a bubble column (0.102 m in ID) operated with a nitrogen-water system. By means of total information entropies IEtotal were identified two transitions: from bubbly flow to transition flow regime and from the latter to the churn-turbulent regime. The effect of axial position z on each of these U-trans values was studied. It was found that both U-trans values were slightly higher in the middle of the column but they decreased in the upper part of the column. In two spouted beds (0.076 and 0.152 m in ID) operated with an air-glass beads system, the total information entropies IEtotal were extracted from gauge pressure fluctuations. It was found that the minimum spouting velocity U-ms in the smaller spouted bed was equal to 0.785 m s(-1), while U-ms decreased down to 0.58 m s(-1) in the bigger spouted bed. The stable spouting regime in the smaller spouted bed covered wider range of gas velocities. (C) 2013 Elsevier Ltd. All rights reserved.