A theoretical and experimental study on the design and performance characteristics of gas inducing impellers is presented. In particular, the model developed by Evans et al. (1991, A.I.Ch.E. Spring National Meeting, Houston, TX, gaper 33e) is critically reviewed and, as a result, improvements to the kinetic energy pressure loss analysis and to the initial conditions are proposed. In addition, the model is successfully extended to account for multiple gas outlet orifice on each blade. Experimental measurements of the power consumption, rate of gas induction, mass transfer coefficient and detached bubble size for a partially optimised, 0.154 m diameter, six-bladed concave gas-inducing impeller are presented. A significant increase in the induced gas rate is observed by adding more outlet orifices to each blade. The principal advantage of using multiple orifices is that similar size bubbles are produced, compared to a single orifice, but larger interfacial areas are generated; the aerated power input is only slightly reduced from its ungassed value. Mass transfer coefficients, k(L)a, of the order of 0.02 s(-1) are attainable for a single outlet orifice on each blade; k(L)a is significantly increased by using multiple orifices. The dimensionless bubble size distributions, d/d(gm) are independent of the impeller speed over the range 4-8 rps, and can be successfully represented by a log-normal distribution.