BACKGROUND: Mixing in unbaffled vessel with multiple-turbine impellers was studied. The mixing time and mixing power were evaluated in relation to the distance between impellers and the number of impellers. RESULTS: It has been confirmed that frequency of oscillation has no influence on the mixing time and mixing power values or on drag and added mass coefficients. The coefficients were greater when distance between impellers was smaller. Moreover added mass coefficient was dependent on Reynolds number (ni > 2). Compared with unidirectional mixing conditions, for systems with one type of impeller, the power requirement was about 38% higher for forward-reverse mixing. Despite the fact that the power demand was greater, the mixing time was not shorter, but about 30% higher than unidirectional mixing in a baffled vessel. However, the forward-reverse mixing mode exhibits a higher level of homogeneity which it achieved faster than unidirectional mixing. CONCLUSION: The power requirements and mixing time for forward-reverse mixing mode were higher in comparison with unidirectional mixing. Despite this, higher values of homogeneity were achieved faster. Higher levels of shear rate and better homogeneity indicate that forward-reverse mixing can be beneficial for multi-phase mixing in vessels with multiple impellers. (c) 2012 Society of Chemical Industry