The paper presents a method for the decentralised solution of the optimal-power-flow (OPF) problem of large, interconnected power systems. The method decomposes the central OPF problem of a multiarea system into independent OPF subproblems, one for each area. The mathematical decomposition method is based on the decoupling of the first-order (KKT) conditions of the original system-wide OPF problem. The solutions of the OPF subproblems of the different areas are co-ordinated through a pricing mechanism until they converge to the system-wide OPF solution. The method requires no parameter tuning for reaching convergence or faster convergence. Results from the application of the method to several IEEE test systems are presented.