The invention relates to a method for determining the gas quality of a sample gas mixed at least in part with biogas, having the main components CH4, CO2, N2, O2, H2, proceeding from a spectrum of the sample gas determined by means of infrared-spectroscopy measurement methods, under operating conditions, from which the mole ratios of the sample gas are determined by means of correlative methods, and converted to characteristic variables of the gas quality. In this connection, the optical absorption of methane CH4 and carbon dioxide CO2 and the heat conductivity λ of the sample gas are measured, the mole ratio xCH4 is determined from the absorption of the CH4, the mole ratio xCO2 is determined from the absorption of the CO2, the mole ratios of nitrogen xN2, of oxygen xO2 and of hydrogen xH2 that are not detected optically are determined from the mole ratios xCH4, xCO2 and the heat conductivity λ, by means of a correlation calculation λ=F(xCH4,xCO2,xN2,xO2,xH2), whereupon characteristic parameters of the sample gas are calculated from the mole ratios obtained in this way.