Experiments were performed in a cylindrical 0.14 m diameter bubble column with a metal perforated plate. Air and aqueous solutions of glycerol with viscosity 1-22 mPa s were the phases. Gas holdup was measured and plotted against the gas flow rate. The critical point where the homogeneous-heterogeneous regime transition begins was determined by the drift-flux plot of the primary data. The homogeneous regime stability was expressed by the critical values of the gas holdup and gas flow rate. The results show that moderate viscosity (3-22 mPa s) destabilizes the homogeneous regime and advance the transition. The results indicate that low viscosity (1-3 mPa s) could stabilize the homogeneous regime. The destabilizing effect of the column height proved previously for air-water system applies also to viscous batches. (C) 2003 Elsevier B.V. All rights reserved.