Determining the gas-transfer index of coal is investigated on the basis of the operation of the PGO-1 laboratory instrument. Its operating principles and construction are briefly described as is the method of measurement of the gas-transfer index. This method of measurement is determined in three successive operations, through use of a vacuum pump to remove all methane and air from the surface, through saturation of the coal with methane to a near-equilibriums state at atmospheric pressure, and through liberation of methane from the coal into the empty space of the instrument with pressure measurement. It is shown that the gas-transfer index of coal is determined not only by the kinetic parameters of desorption but also by the technology used in measuring this index which is described by the mathematical model developed here for the pressure rise in a closed volume on account of the desorbed gas.