A forced circulation pumped loop reactor is characterized for oxygen transfer in air-water system. Overall mass transfer coefficient (k(L)a) data are reported for airlift and forced circulation modes of operation, for liquid circulation rates of up to 2 m(3)/h. Highest values of oxygen transfer efficiency were attained at specific power input values of <= 100 W/m(3) when the forced circulation rates were <= 0.5 m(3)/h. Higher values of forced circulation rate reduced mass transfer efficiency, but reactor was always more efficient than a propeller loop reactor. The kl.a values obtained at high rates of pumped liquid circulation were substantially greater than could be attained in the airlift mode of operation. Forced circulation produced more uniform and small bubbles, compared to operation as an airlift reactor. At high rates of forced circulation (e.g. 2 m(3)/h), presence of relatively light (density = 931.8 kg/m(3)) suspended hydrophobic polypropylene particles (average diameter = 4.7 mm) at concentrations of 1.6 and 3.2% (v/v), barely affected k(L)a compared to solids-free operation. The reactor used had an aspect ratio of 6.2 and downcomer-to-riser cross-sectional area ratio of 0.032. The forced flow was injected in the annular riser zone. (c) 2005 Elsevier B.V. All rights reserved.