Gas entrainment into centrifugal pumps decreases pump performance and may raise safety issues, e.g., through insufficient cooling. Although there is some phenomenological knowledge in the form of correlations between operating parameters and pump performance, a further understanding via direct observation of the gas-liquid mixture was so far not possible. In this paper, we demonstrate the capability of ultrafast X-ray computed tomography (UFXCT) to disclose gas-liquid two-phase flow dynamics in the impeller region of a centrifugal pump mockup. Experiments were performed for gas injection at impeller speeds between 1300 rpm and 1600 rpm. We analyzed the X-ray image sequences with respect to characteristics of the gas distribution and compared them with time-Averaged image data of a real pump obtained earlier with gamma-ray computed tomography (CT).