A re-entrant resonator has been developed and tested, specifically tailored for automated phase boundary measurements in the natural gas fluids of the north west shelf of Western Australia. The resonator is based on a constant volume, rf re-entrant resonant cavity capable of detecting phase boundaries in binary mixtures [1]. Our resonator is capable of isothermal and isobaric measurements in addition to isochoric measurements. The vacuum characteristics of the resonator have been studied using extensive models, and measured in experiments with helium. The resonator was then employed to study phase transitions in pure carbon dioxide and a binary mixture of propane and carbon dioxide. The suitability of the resonator for dew point detection in lean hydrocarbon mixtures was also tested. The results indicate that microwave resonators are not just limited to phase boundary measurements, rather they can measure phase volumes and potentially determine the compositions of the two-phases. However, further development is needed to achieve a system in which accurate measurements can be made for gas condensate type fluids.