The problem of input-output decoupling and simultaneous pole assignment for generalised state-space systems via pure proportional-state feedback is studied. A new design approach is presented for the derivation of the general analytical expressions of the decoupling controller matrixes and of the diagonal elements of the transfer function of the decoupled closed-loop system. On the basis of this approach, the first results on the structural properties of the closed-loop system are established covering the arbitrariness of the poles of the general form of the transfer function of the decoupled closed-loop system, the necessary and sufficient conditions for simulta neously decoupling, elimination of impulsive behaviour and arbitrary allocation of the poles of the characteristic polynomial of the closed-loop system, and the analytical determination of the poles which are cancelled out in the general form of the transfer function of the closed-loop system. The results derived are applied for the decoupling and pole assignment of a nuclear reactor plant.