Gas-flammability sensing systems and methods may be used to determine the flammability of gas mixtures in measurement volumes such as a fuel tank (e.g., an aircraft fuel tank). Gas-flammability sensing systems include a test cell structured to receive a gas sample, a heater in thermal communication with the test cell, and a gas meter configured to measure a physical property of the gas sample within the test cell related to the combustion state of the gas sample. The heater is configured to heat the gas sample to an elevated temperature less than the autoignition temperature of the gas sample. Methods of determining the flammability of a gas sample include collecting the gas sample, heating the gas sample to the elevated temperature, measuring the physical property of the gas sample after heating, and determining the flammability of a gas sample based upon the measured physical property.