The motivations and main results of the research activities in the field of mechanically agitated aerated vessels done at Prague Institute of Chemical Technology are presented. The first period of our research work was devoted to finding the methods, which provide physically correct k_La values. Because most of k_La-data presented in literature have been measured in laboratory scale apparatuses, our further research has been focused on finding proper correlations, which would enable the usage of the laboratory results for industrial vessels design. Hence, a new method for physically correct k_La-data measurement, applicable also in large vessels, was developed. Our recent results obtained in a pilot-plant vessel of inner diameter 0.6 m equipped with three Rushton Turbines are presented. The physical correctness of the k_La values is confirmed by the agreement of the k_La values measured by air with those measured by pure oxygen absorption. Their consistency confirms a correct separation of the mass transfer coefficient from the driving force of the absorption. Retrieved k_La values were compared with previous experimental results obtained using laboratory scale vessel of i.d. 0.3 m. The pilot-plant vessel provides k_La values that are 25% lower at the same impeller power per cubic meter and the same superficial gas velocity. Other process parameters, which would improve the accuracy in the maintenance of the desired k_La values in the scaling- up procedure, were sought. The more precise correlations, based on the data from both the laboratory and the pilot-plant scale vessels have been established.