Compositions of natural gases are different between each other because of different sources, resulting in the fact that natural gases have different energy contents and monetary value. This paper presents a theory to monitor the combustion properties of natural gas by using the acoustic relaxation phenomenon in which the maximum point of acoustic spectrum varies with gas composition. The theory is developed from the frequency-dependent sound absorption spectrum which can be reconstructed from its maximum point synthesized in the acoustic measurements at two frequencies. The theory uses the relation between the two values of the maximum point (i.e. the relaxation frequency and the maximum relaxational absorption) and gas composition to quantitatively monitor the gas. Moreover, the theory has the advantages of avoiding the detection of the gas density and the variation of the ambient pressure, which is necessary in the traditional way of measuring the maximum point of sound absorption spectrum.