This paper compares the kinetic behaviour of a given chemical reaction in homogeneous and heterogeneous polymer melts using a batch mixer as reactor. The underlying chemistry was the addition esterification of epoxy groups attached to a polymer backbone and a carboxylic acid. Specifically, the homogeneous polymer melt was composed of 1-naphthylacetic acid (NAA) and a random copolymer of ethylene, ethyl acrylate and glycidyl methacrylate (CoGMA). The organic acid was soluble in the CoGMA melt under the reaction conditions. The heterogeneous polymer melt was the aforementioned homogeneous polymer melt to which was added an inert polymer, polystyrene (PS). The PS and the CoGMA were immiscible while NAA was miscible with both polymers. Experimental results showed that the overall reaction rate in the homogeneous polymer melt was controlled by its intrinsic kinetics only while that in the heterogeneous polymer melt was dictated not only by the intrinsic kinetics, which is the same as that in the homogeneous polymer melt, but also by the partition coefficient of NAA between the CoGMA and the PS phases. These results can be justified by the fact that the rate of diffusion of NAA is much greater than the rate of reaction for both homogeneous and heterogeneous polymer melt systems, as mechanical mixing was necessary only at the initial stage of the reaction.