An apparatus based on a microwave resonant cavity has been designed and fabricated for phase behaviour measurements in lean gas condensate fluids over a wide range of temperature and pressure. The re-entrant geometry of the resonator is optimised for detecting any liquid phase present in very lean natural gases. The mode structure of the cavity has been thoroughly investigated with both analytic and finite element models. Three modes, excited by an electric field probe, are monitored when measuring a fluid contained within the resonator. The highest mode (f(vac) approximate to 6.9 GHz) is used to detect dew points while the lower modes (460 MHz and 4.3 GHz) are employed for liquid volume and dielectric constant measurements. Careful microwave circuit design ensures good signal-to-noise ratios for all modes over the operating temperature range. Simulations of the resonator containing various fluids indicate that the system is over 104 times more sensitive than previous microwave systems and can detect liquid volumes as small as 5 x 10(-6) cm(3). Dew point measurements in a gas condensate fluid support this prediction. (C) 2003 Elsevier B.V. All rights reserved.