It is known that a head breakdown of centrifugal pumps in a two-phase flow occurs at a gas/liquid ratio less than ten percent, due to gas accumulations in the impeller. If these accumulations could be evacuated by use of an impeller with multiple tandem blades, two-phase flow pumps might be operated at a higher gas/liquid ratio. Tests to examine the effects of a tandem-bladed impeller on the pump performance in single-phase and two-phase water/ air flows are described. Experimental results in comparison with results of an unsplit impeller thow that there is no difference of the pump performance in a single-phase flow between both impellers, and a two-phase head degradation in a flow region with elongated bubbles is improved at large water flow rates.