In recent years it has been an attention to dielectric response parameters for diagnosis of power transformers, high voltage cables etc. Determination of water content of pressboard, and aging state of solid insulation materials are claimed and some commercial measuring systems for these measurements are available. In this paper some measurement on oil-pressboard samples in laboratory condition at different states of samples are presented. The aim of the investigation is the determination of the effect of some parameters such as aging, water contents, geometry, etc on the dielectric response. The results show that separation of oil quality and pressboard state is possible and that aging has a similar effect as moisture.