Abstract: The coupling reaction of α-naphthol with diazotized sulphanilic acid was used as a working reaction system for checking the effectiveness of the dual-sphere model in a stirred batch reactor. Experimental results showed that the model gives a better fit than Rys model. Experiments also showed that the working reaction system, together with the present model, can be applied as a measure for determining the mean size of sub-elements in an agitated reactor. On the basis of the size determined, a basis for scaling up agitated reactors was established.