The use of Airey's emission constants (n and t 0) and the new Soviet quantity sorption-kinetic parameter (τ) for characterising the emission of various gases from coal and adjacent strata samples are compared. It is shown that it is more desirable to use the parameter τ, which has previously been shown to be a reliable characteristic for sorption-kinetic experiments involving carbonaceous materials. Comparison of τ with Airey's time constant t 0 for samples having differing values of emission index n indicates that they are similar (variation ± 0.5%) for samples where n = 0.5, whilst there is a strong difference between them for higher or lower values of n. Calculations for both methane and ethane desorption data from samples with a wide variation in particle size show that the parameter τ is a reliable characteristic of gas emission from coal of any particle size. The results for both gases were similar; hence the same method of calculation is applicable in both cases. It is demonstrated that whilst the speed of gas desorption decreases with decreasing equilibrium gas pressure, the proportion of gas desorbed in time τ is relatively constant for samples exhibiting similar values of Airey's emission index n.