Deterioration of post-tensioned structures may take place as a consequence of severe environmental conditions or design or construction errors. This paper investigates analytically the effects of corrosion on the structural response of bonded post-tensioned beams. An experimental study on the stress corrosion of prestressing wires is described first, providing data on the steel behaviour. A series of tests on beams taken from the literature is then analysed. The breaking of a part of the wires was caused by artificial corrosion; the beams showed stiffness and strength deterioration as a consequence of further breakages occurring under imposed load. Two different analytical approaches are used: macro finite-element analysis based on the calculation of the cross-section response and non-linear finite-element analysis. The analyses provide the global response and local strain values with the interpretation of the causes of the deterioration of the response.