Design and operation of unbaffled aerated agitated vessels with multiple unsteadily forward-reverse rotating impellers (AJITERs) for viscous Newtonian liquids were studied. The effects of operating conditions such as gas sparging rate, agitation rate and the number of impeller stages, geometrical conditions such as the diameters of vessel and impeller, and the physical properties of liquids on the drag and added moment of inertia coefficients, necessary to predict the average and maximum power consumptions of the impellers in AJITERs, were evaluated and the empirical relationships which estimate values of each of these coefficients are presented. The effects of operating conditions, geometrical conditions and liquid physical properties on the gas hold-up, phi(gD), and volumetric oxygen transfer coefficient, k(L)a(D), were evaluated in relation to the total power input which is the sum of the average power consumption of impellers, ie average agitation power input, and aeration power input. Empirical relationships, useful for design and operation of AJITERs, were obtained for each viscosity range, where the dependences of phi(gD) and k(L)a(D) on the specific total power input and superficial gas velocity differed, to predict phi(gD) and k(L)a(D) respectively as a function of the specific total power input, superficial gas velocity and liquid physical properties. (C) 2003 Society of Chemical Industry.