The possibility of prediction of the gas hold-up for mechanically agitated gas-liquid systems from a theoretical equation proposed by Kudrewizki [3] has been analyzed in the paper. The calculations have been carried out for coalescing and non-coalescing systems, agitated in the vessel of the inner diameter D equal to 0.634 m and equipped with a Smith impeller or a turbine with flat blades. The results of the computations have been compared with our experimental data. The results of the scale-up effect on the experimentally obtained process characteristics for mechanically agitated gas-liquid systems have also been presented. The measurements were carried out in four agitated vessels equipped with single or dual impellers and differing ten times in the working volume of the fluid. The scale-up effect on the power consumption, gas hold-up and residence time of the gas bubbles in the system has been estimated.