The interest in the process industry on unbaffled stirred tanks has greatly expanded in the last years because they may bring about significant advantages in a number of applications, including biochemical, food and pharmaceutical processes where the presence of baffles is undesirable for several reasons. Despite their application potential, unbaffled vessels still lack fundamental information, due to the fact that only recently their capabilities have started being dug out. The lack of information on scale up effects is possibly the main reason hindering practical applications. In this work the influence of vessel size and liquid viscosity on the mass transfer performance in unbaffled stirred vessels, is investigated. As regards the first issue (scale-up) quite surprisingly results show that comparable mass transfer coefficients may be obtained in larger vessels, for a given power input per unit volume, with typical k(L)a values up to 2 x 10(-3) s(-1) in ungassed conditions and of 2 x 10(-2) s(-1) in gassed conditions. This is opposite to expectations based on the fact that the exchange surface in subcritical conditions only grows with D-2 while volume grows with D-3. Notably, the same result is obtained at an increased liquid viscosity (a feature often exhibited by biological cultivation broths) and confirms that unbaffled stirred tanks should be regarded as a strong candidate for industrial applications. Finally, also at the larger scale, in the super-critical regime unbaffled tanks are found to provide a performance comparable with that of standard (baffled & sparged) stirred tanks, hence resulting in a viable alternative to baffled tanks for all gas-liquid processes and bio-processes. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.