The void fraction in the boiling layer of liquefied natural gas is one of the parameters used to characterize the cryogen-water mixture when formulating predictive models for rapid phase transitions (RPT). With the experimental apparatus described herein for measuring the void fraction in boiling hydrocarbon mixtures, the calibration point for single-phase liquid was not possible to obtain directly. In addition, the composition of a boiling mixture changes continuously. Two different-methods are described for the estimation of the chemical composition, and thereby the liquid properties. The results showed little sensitivity to the liquid properties, regarding the void fraction value.