The degradation (fatigue) of dielectric properties of ferroelectric PZT (Lead Zirconate Titanate) thin films during cycling was investigated. PZT thin films were fabricated by metal-organic decomposition (MOD). Samples with electrodes of platinum (Pt) and ruthenium oxide (RuO2) were studied. The interfacial capacitance (if any) at the Pt/PZT and RuO2/PZT interfaces was determined from the thickness dependence of low-field dielectric permittivity (εr) measurements. It was observed that a low εrlayer existed at the Pt/PZT interface but not at the RuO2/PZT interface. The dielectric permittivity of this interfacial layer degrades with increasing fatigue while the εrof the bulk PZT film remains constant. This indicates that fatigue increases the interfacial layer thickness but does not change the bulk properties. For the capacitors with RuO2/PZT/RuO2 structure, however, the εdoes not change with thickness and fatigue cycling. This implies no interfacial layer exists between RuO2/PZT and, therefore, no fatigue was observed. Additionally, an impedance spectroscopie technique has been proposed for possible use in analyzing the nature of the interfacial layer during the fatigue process.