Abstract: A method detects the proportion of combustible gas in a mixture of air with a measuring device which operates by both the heat-tone and heat-conductivity methods. The method permits measurement in the full concentration range with a high degree of accuracy and the lowest possible energy consumption. The measuring device is made small and light and its sensor has a long service life. The method uses one single sensor which operates at a low temperature in the heat-conductivity mode or alternatively, catalytically active at a higher temperature in the heat-tone mode. If the measurement provides a result below a reference value or within a reference value window, measurement continues in the more accurate heat-tone mode. When the gas concentration increases during measurement, in order to avoid ambiguity of the measuring signal and damage to the sensor above a load-limit value, measurement again takes place in the heat-conductivity mode by reducing the temperature of the sensor.