Airlift pumps are the simplest devices used for lifting and transporting of liquid in water and wastewater systems. They constitute the subject of interest in numerous studies, focusing on their two-phase flow. The knowledge of two-phase flow parameters is necessary for the correct design of an airlift pump. In the available literature on the subject, one will find numerous models describing the two-phase gas-liquid flow. One of the most popular is the Zuber – Findlay model, also called the slip model. The subject of the present research consisted in a vertical bubble column, with the diameters of 50 and 75 mm, used to transport the water-air mixture. Based on the results of the laboratory experiments the hydraulic characteristics were developed: liquid flow depending on the depth H_s and the air flow Q_p. For the mathematical description, second-degree polynomial model was applied, as well as and assessment of the determination coefficient was performed. Efficiency factors of the airlift pump and other parameters for two-phase mixture during the airlift pump’s operation were determined using the slip model. Based on the experiment, it has been found for both of the analysed diameters that the efficiency of the airlift pump increased along with the depth H_s and the air flow Q_p. The airlift pump achieves maximum liquid efficiency within a specified range of the immersion depth. In a small diameter airlift pump (50 mm), slug and plug flow regimes are predominant.