Bubble formation at two, three, four and six symmetrical orifices has been investigated by high-speed photography and analysis of chamber pressure fluctuations. Regimes of synchronous, alternative and unsteady bubbling were clearly identified, and the effects of orifice spacing and liquid depth on bubbling synchronicity and frequency were studied. Theoretical predictions of bubble frequency and average radius were found to be in good agreement with experimental data when the bubble regime was highly synchronous. (C) 2003 Elsevier Ltd. All rights reserved.