Gas hold-up and oxygen transfer have been investigated in two geometrically similar external loop airlift reactors of linear scale ratio of 2. In mass transfer experiments, the sampling location was found to be important as significantly different k(L)a values can be obtained. The variations of k(L)a with probe location have been explained in terms of non-uniform hydrodynamic properties and the results obtained have been validated by means of high speed video camera recordings. At higher gas flowrates, the gas hold-up was significantly higher in the large-scale reactor. It was found that in order to maintain the gas hold-up or k,a constant in both the small- and large-scale reactor, the small-scale reactor required 25% and 27% more power input per unit volume of liquid respectively.