The invention relates to an improved and simpler method and to an apparatus for the determination of the CO2 emission factor in flare gas plants comprising the steps: a) measuring the speed of sound in the flare gas; b) detecting known proportions of N2, CO2 and H2O; c1) calculating a maximum speed of sound of the flare gas under the assumption that the hydrocarbon proportion of the flare gas only consists of alkanes having a chain length i; c2) calculating a minimal speed of sound of the flare gas under the assumption that the hydrocarbon proportion of the flare gas only consists of alkanes having a chain length i+1; c3) varying the chain length i for so long until the measured speed of sound lies between the calculated minimal speed of sound and the calculated maximum speed of sound; d) varying the proportions of the alkanes having the chain length i and the chain length i+1 determined in step c) for so long until the speed of sound calculated with these proportions lies within a predefined difference from the measured speed of sound; e) calculating the equivalent chain length; and f) calculating the emission factor.