The performance of internal-loop airlift bioreactors is affected by their geometry, especially the bottom and gas-liquid separator designs. Despite efforts to understand the impact of geometry on bioreactor hydrodynamics and oxygen transfer, the contribution of each region to the bioreactor performance as a whole has been evaluated separately. Consequently, it has not been possible to define the best overall geometry. This work discusses the influence of the bottom and gas-liquid separator geometries on the volumetric oxygen transfer coefficient (k(L)a) and superficial liquid circulation velocity (U-L) of 10-L square cross-section concentric-duct airlift (CDA) and split airlift (SA) reactors. Both airlift reactors were operated with distilled water and Saccharomyces cerevisiae cultivation broth. Based on the results it was possible to indicate the best geometric configurations for biotechnological applications. Gas-liquid separator design was evaluated by changing the openness angle (alpha) and the gas-liquid separator volumetric liquid fraction (F-GLS), while the free area for liquid flow between riser and downcomer (A(b)) was the geometric parameter modified for the bottom. The results showed that liquid circulation was strongly affected by the bottom geometry, while the gas-liquid separator significantly affected both variables. Combining the impacts of both regions, promising airlift geometries were identified that exhibited low or high hydrodynamics and oxygen transfer levels. These findings highlight the flexibility of airlift bioreactors and provide information required for their design in order to satisfy specific requirements of different aerobic bioprocesses.