Experiments were conducted to study the effect of the presence of the solid phase on the homogeneous-heterogeneous flow regime transition in a bubble column 0.14 m diameter. Air, distilled water and calcium alginate beads (2.1 mm, 1023 kg/m(3)) at concentrations c = 0-30% (vol.) were the phases. The basic data were the voidage-gas flow rate dependences. The critical point, where the homogeneous regime loses stability and the transition begins, was evaluated by the drift flux model. The critical values of voidage and gas flow rate were the quantitative measures of the homogeneous regime stability. These were plotted against the solid phase concentration. It was found, that both the voidage and the critical values increased with the solid content at low solid loading, approx. c = 0-3 %, and decreased at higher loading, c > 3%. The homogeneous regime was thus first stabilized and then destabilized. To explain this dual effect, possible physical mechanisms of the solid phase influence on the uniform bubble bed were discussed. (c) 2005 Elsevier Ltd. All rights reserved.