The behaviour of gas-liquid mixtures in the vicinity of the blades of an unsteadily rotating impeller in an unbaffled agitated vessel was studied by observations made with a rotating camera. The impellers used were a disk turbine impeller with six flat blades (DT) and a novel cross-type impeller with four delta blades (CD). The behaviour of gas-liquid mixtures near the blades of the forward-reverse rotating impeller was unsteady in terms of the formation of cavities behind the blades and their dispersion into gas bubbles, and differed from that near the blades of a unidirectionally, steadily rotating impeller. The differences in relative power consumption between the forward-reverse rotating impellers in the unbaffled vessel and the steadily rotating impellers in the baffled vessel are discussed in relation to the differences in the behaviour of gas-liquid mixtures near the blades of each rotating impeller.