This paper presents numerical simulations and experimental studies on the fluid-dynamic behaviour of rising bubbles in stagnant and flowing liquids in a vertical straight duct of rectangular cross-section. The bubbles were generated by injecting air through an orifice of the side-wall. Experimental data were obtained for various liquid flow velocities in order to assess the impact of the stream conditions on the bubble characteristics. The bubble size, their rising behaviour as well as the bubble velocity were investigated systematically using high-speed cinematogaphy. The velocity field in the vicinity and the wake of the rising bubbles was measured by means of the Laser Doppler velocimetry to quantify the local structure of the liquid flow in the presence of rising bubbles. According to the experimental configuration and the obtained data numerical calculations have been performed. For these numerical computations, a model for the bubbles was added to a solver for the incompressible Navier-Stokes equations. This model captures the motion of the bubbles, yet omits details of their shape. The results of the experimental study show the complexity of the bubble dynamics in superimposed liquid flow. Experimental data obtained for stagnant liquid compare well with the numerical predictions.