In this paper, the flow regimes developed in a centrally aerated two-dimensional (2D) bubble column are studied by means of visual observations and the measurement of wall pressure fluctuations. The combined effect of the aspect ratio (H/W) and superficial gas velocity (UG) on the hydrodynamics of the bubble column is investigated. Moreover, the time-averaged and time-dependent flow behavior are analyzed together. The results reveal the importance of H/W and UG on the developed flow regimes and the significance of the steadiness or unsteadiness of the flow on the resulting time- averaged flow regimes. A detailed description of the flow patterns based on H/W and UG is presented. As UG is increased, at H/W = 1.25, two pseudo-steady-state flow regimes (characterized by one or two circulation cells) are observed. At H/W = 2.25, only unsteady bubble plumes of decreasing oscillation period are obtained. At H/W = 1.50, 1.75, and 2.00, unsteady flow structures occur, at increasing values of UG.