The influence of the process parameters on the oxygen transfer rate in the external-loop airlift reactor with the inverse fluidized bed in the downcomer has been investigated. Dependences between the oxygen transfer coefficients in the airlift reactor, external-loop airlift reactors with the fluidized bed in the riser, the suspension airlift reactor and the bubble column are discussed. The contribution of microbubbles in the gas-liquid mass transfer has been estimated.