The objective of this study is to improve mass transfer models in mechanically-agitated-contactors (MAC). The mass transfer is predicted by using a two-phase two-dimensional computational fluid dynamics (CFD) approach. A two-fluid model for two-phase dispersed flow in low gas volume fraction MAC is presented herein. Emphasis is placed on the closure correlations used in this study. Different drag coefficients and slip velocity correlations for dispersed bubbly flows quoted in the literature are studied and compared. Boundary conditions are discussed at the air injection point and impeller. Simulation results are compared to the experimental data for overall volumetric mass transfer coefficients k(L)a; satisfactory agreement is found on a wide range of operating conditions. The experiments were performed in a 0.54 m(3) cylindrical, non-standard, one-stage reactor, with a flat bottom equipped with an axial impeller.