A detailed study of the effects of the ageing on the characteristic parameters of polyaniline/p-type SUM structure has been presented. The polyaniline film has been formed on a p-type Si substrate by means of an anodization process. The polyaniline/p-Si/Al structure has demonstrated clearly rectifying behavior by the current-voltage (I-V curves studied at room temperature. The current-voltage curves of the structure have been measured immediately, 15, 30, 60, 90 and 120 days after fabrication of the polyaniline/p-Si/Al structure. It has been seen that the characteristic parameters, such as barrier height (BH), ideality factor and series resistance of polyaniline/p-type Si/Al structure have slowly changed with increasing ageing time. The diode shows nonideal I-V behavior with an ideality factor greater than unity that can be ascribed to the interfacial layer, the interface states and the series resistance. (C) 2004 Elsevier B.V. All rights reserved.