In this study, the liquid side volumetric mass transfer coefficient k(L)a was determined at the optimal condition for mechanically agitated reactors in which gas/liquid mass transfer occurs. For this purpose the ’experimental design’ technique was utilized. A mathematical model was built up in the form of linear and nonlinear algebraic equations and the optimal values of the k(L)a and of relevant parameters were computed through the application of the Box-Wilson technique for linear models. To determine the values of the k,a and the parameters which affect k(L)a in optimal working conditions in the unsteady state, the adaptive optimization algorithm has been used. Surface plots of the optimal values obtained from the nonlinear model were also presented.