The liquid–solid mass transfer behaviour of a fixed bed of Raschig rings placed in the riser or the downcomer of a rectangular airlift reactor was studied by a technique involving the diffusion controlled dissolution of copper in acidified dichromate. Variables studied were superficial gas velocity, ring diameter, bed height and physical properties of the solution. The mass transfer coefficient was found to increase with the 0.425 exponent of the superficial gas velocity, at relatively high superficial gas velocity a limiting mass transfer coefficient was reached. The mass transfer coefficient was found to decrease slightly with increasing bed height, ring diameter was found to have a little effect on the rate of mass transfer. The data were correlated for fixed beds placed in the riser and fixed bed placed in the downcomer by dimensionless equations involving the groups J, Re and Fr. Comparison of the present data with previous studies on sparged fixed beds confirmed the superiority of packed bed airlift reactors to packed bubble column reactors. A natural convection mathematical model was found to explain the mechanism of mass transfer at the fixed bed airlift reactor.