The present work focuses on the determination of gas holdup, Sauter mean bubble diameter (d(32)) and gas-liquid interfacial area (alpha(g-1)) for radial- and mixed-flow impellers under various operating conditions in a three-phase stirred vessel. d(32) values were determined for a wide range of solids concentration (0.01-0.25 v/v) using an underwater imaging technique when particles are just suspended off the tank bottom. The results indicated that d(32) values decrease with an increase in particle size and solids concentration at a constant gas flow rate. It was observed that process intensification is an effective strategy for improving the interfacial area between the gas and liquid phases in a multi-phase stirred tank agitated by a radial-flow impeller. Considering the effect of solids concentration on impeller specific power, an optimum solids concentration is identified at which the performance of the impeller expressed in terms of power efficiency and ability to generate sufficient gas-liquid interfacial area is maximized. Mathematical correlations to estimate d(32) and alpha(g-1) in gas-liquid-solid stirred vessels are proposed. (c) 2016 Elsevier B.V. All rights reserved.