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The paper is concerned with the use of an open-ended coaxial-line probe for making dielectric measurements on lossy materials. The input reflection coefficient of the probe is measured with a network analyser, and the permittivity is determined using an approximate circuit model for the probe admittance. Applying the cross-ratio invariance property of the bilinear transform to two such circuit models, we derive a simplified procedure that permits simultaneous calibration of the probe and error correction of the network analyser. Estimates of the resulting uncertainties are given, and the technique is illustrated with experimental measurements on known dielectric liquids between 50 MHz and 2.6 GHz.