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Direct measurements of impurity freeze-out conditions in real time are needed to improve the reliability of liquefied natural gas (LNG) production. Benzene and carbon dioxide are two impurities in the feed mixtures processed by LNG plants that can freeze, deposit and block cryogenic heat exchangers, even at trace concentrations. Existing technologies for monitoring the risk of impurity freeze out, such as compositional analysis, are inherently indirect and of limited resolution and accuracy. Here, we present a window-free, microwave sensor capable of measuring the freeze-out conditions of low-concentration impurities in high-pressure natural gas mixtures at cryogenic temperatures. The sensor is designed so that solids freezing out from the high-pressure LNG mixture are located in a region of high electric field strength, causing resonance frequency shifts of order 0.03 MHz per nanolitre of solid benzene. Finite-element models of the sensor performance were validated by measuring the freeze-out of benzene and carbon dioxide solutes at (50 to 100) parts-per-million in liquid methane at pressures from (7 to 9.5) MPa and temperatures from (90 to 150) K. The operational use of such a sensor has significant potential for improving the reliability of LNG production and identifying blockage remediation options that avoid complete plant shut downs.