For a gas-liquid contactor equipped with two Rushton disk turbines (DT), where the upper DT is located very near the static liquid surface, we have determined experimentally its volumetric mass-transfer parameter k(L)a as a function of the power input, P/V. It is interestingly found that in such a contactor multiple steady states occur, i.e., for a given P/V value, two k(L)a values exist or vice versa in the region of low P/V, leading to two (lower and upper) branches in the k(L)a vs. P/V plot. Detailed investigations indicate that the lower branch represents the mass transfer behavior of typical gas-liquid contactors with two DTs in the literature, while the upper branch results from a different mass transfer mechanism. We believe that presence of the upper branch is because of the small distance of the upper DT to the static liquid surface, which in the range of low P/V values enhances the surface gas entrainment, leading to substantial difference in the local gas holdup between the upper and lower DTs. It is this inhomogeneity that generates the multiplicity phenomenon. (C) 2011 Elsevier B.V. All rights reserved.