For a natural gas and, especially, retrograde condensates, it is important for exploration and production that the (liquid + gas) phase boundary be known along with the ratio of liquid-to-gas volumes within the (liquid + gas) two-phase region. These fluid properties can be measured by a plethora of methods and here we report a method based on the measurement of the resonance frequency of the lowest order inductive-capacitance mode of a re-entrant cavity capable of operating at temperatures up to 473 K and pressures below 20 MPa. This instrument has been used to measure, at T < 340 K, the dew pressures of {0.4026CH(4) + 0.5974C(3)H(8)}. The measured dew pressures differ by less than 0.5% from values obtained by interpolation of those reported in the literature, which were determined from measurements with experimental techniques that suffer from quite different potential sources of systematic error than the radio-frequency resonator used here. Dew pressures estimated from both NIST 14 and the Peng-Robinson equation of state lie within <+/- 1% of our results at temperature between (315 and 337) K while predictions obtained from the Soave Redlich Kwong cubic equation of state deviate from our results by 0.4% at T=315 K and these absolute differences increase smoothly with increasing temperature to be -2.4% at T=337 K. (c) 2004 Elsevier Ltd. All rights reserved.