Gas-liquid mass transfer was studied experimentally for an air-water system in an unbaffled vessel with a liquid height-to-diameter ratio of 2:1 that was agitated by unsteadily forward-reverse rotating multiple impellers which alternated rotation direction. Two kinds of four-bladed impellers were used with different designs or triangular delta blades. One (S impeller) has a straight blade with a downward triangle apex throughout. The other (T impeller) has a twisted blade composed of an inner downward part and an outer upward part. With both the impellers, enhancement of the mass transfer was observed with increases in the liquid-phase mass transfer coefficient as well as the gas-liquid interfacial area that determine the magnitude of the volumetric coefficient characterizing the rate for water to absorb oxygen in air. The respective dependences or mass transfer parameters on the axial position within the vessel were evaluated for the mean bubble diameter and gas hold-up. T impeller produced a gas-liquid interface with a larger area and more uniform distribution than S impeller. The effect of forward-reverse agitation using the delta blade impellers on formation of the gas-liquid dispersion was confirmed in relation to the behaviour or gas bubbles within the vessel.