This paper describes the effect of rotor radius, rotor-stator distance, liquid flow rate and rotational disc speed on the rate of gas-liquid mass transfer in a rotor-stator spinning disc reactor. A rotor radius of 0.135m is studied with rotor-stator distances of 1, 2 and 5 mm, at rotational disc speeds up to 209 rad s(-1), and compared with a rotor radius of 0.066 m. At rotational disc speeds lower than 70 rad s(-1), elongated gas bubbles are formed, that are larger than the rotor-stator distance. At rotational disc speeds above 100 rad s(-1), spherical gas bubbles are formed that are smaller than the rotor-stator distance. The volumetric gas-liquid mass transfer coefficient increases with increasing rotational disc speed and decreases with increasing liquid flow rate. This decrease is larger than predicted by the Walks drift flux model because of the complex two-phase flow pattern. The rate of gas-liquid mass transfer per unit of reactor volume increases with decreasing rotor-stator distance. The maximum observed volumetric mass transfer coefficient in case of the 0.135 m rotor is a factor 3 higher than in case of the 0.066m rotor, while the rate of energy dissipation is a factor 15 higher. (C) 2011 Elsevier B.V. All rights reserved.