BACKGROUND: The bubble size distribution in gas-liquid reactors influences gas holdup, residence time distribution, and gas-liquid interfacial area for mass transfer. This work reports on the effects of independently varied gas and liquid flow rates on steady-state bubble size distributions in a new design of forced circulation loop reactor operated with an air-water system. The reactor consisted of a cylindrical vessel (similar to 26 L nominal volume, gas-free aspect ratio approximate to 6, downcomer-to-riser cross-sectional area ratio of 0.493) with a concentric draft tube and an annular riser zone. Both gas and liquid were in forced flow through a sparger that had been designed for minimizing the bubble size. RESULTS: Photographically measured bubble size distributions in the riser zone could be approximated as normal distributions for the combinations of gas and liquid flow rates used. This contrasted with other kinds of size distributions (e.g. bimodal, Gaussian) that have been reported for other types of gas-liquid reactors. Most of the bubbles were in the 3 to 5 mm diameter range. At any fixed low value of aeration rate (<= 1.8 x 10(-4) m 3 s(-1)), increase in the liquid flow rate caused earlier detachment of bubbles from the sparger holes to reduce the Sauter mean bubble size in the riser region. CONCLUSION: Unlike in conventional bubble columns where bimodal and Gaussian bubble size distributions have been reported, a normal bubble size distribution is attained in forced circulation loop reactors with an air-water system over the entire range of operation. (c) 2007 Society of Chemical Industry.