The invention relates to a method and a device to determine the gas properties of combustible gas, in particular of natural gas, wherein a) at least part of the combustible gas is exposed to infrared radiation and the amount of infrared radiation absorbed by the combustible gas is determined for two wave lengths or spectral ranges, the two wave lengths or spectral ranges being selected so that the amounts of individual components of the combustible gas in their different percentages have an effect on the amounts absorbed and recorded, b) the thermal conductivity is recorded and c) the gas properties are determined from the three measurands. The term gas properties is understood to mean the gas composition, the gross heating value, the Wobbe number, the normal density and the methane number.