The incommensurability thesis, as introduced by T.S. Kuhn and P.K. Feyerabend, states that incommensurable theories are conceptually incompatible theories which share a common domain of application. Such claim has often been regarded as incoherent, since it has been understood that the determination of a common domain of application at least requires a certain degree of conceptual compatibility between the theories. The purpose of this work is to contribute to the defense of the notion of local or gradual incommensurability, as proposed (...) by late Kuhn. The application of this notion would allow to render the incommensurability thesis coherent. To support this view, a typical example of incommensurability will be formally analyzed by applying the structuralist metatheory developed, among others by W. Balzer, C.U. Moulines and J.D. Sneed. The structural reconstruction of the relation between the phlogiston theory and the oxygen theory offered here will reveal that they are locally incommensurable, and will even make possible to determine the ontological reduction relation that they also exemplify. (shrink)