The combination of a solid and a liquid insulation is the most frequently used insulating system in high voltage apparatus, where components have to be insulated and loss heat has to be dissipated. The requirements on the liquid part of the insulating system are not only the electric and dielectric performance but also the performance regarding environmental requirements and dehydration capability as well as low inflammability. The use of ester liquid, partly or totally replacing mineral oil, reduces the risk of environmental pollution, increases the lifetime of the component and reduces the fire risk. Furthermore the capability of ester liquid to assimilate a higher amount of water in comparison to mineral oil without reducing its electric strength reduces the breakdown endangerment during cold start and allows the operation of the component at higher temperatures. The contribution presents a comparison of the performance of ester liquid and mineral oil as well as oil/ester mixtures concerning the electric behaviour and covers the whole range of blends between ester liquid and mineral oil. The dielectric parameters like dissipation factor and permittivity have been measured for the whole mixture range in order to evaluate the performance of the complete insulating system regarding the compatibility of the liquid insulation with different types of paper.