The method of determining, by absorption spectroscopy, the pressure of a gas mixture in a vacuum container located in an environment containing the gas to be measured, and the pressure of the gas is different from that in the vacuum container. A device comprising a laser and a laser detector measures an absorption curve A.sub.R(.lamda.) of the gas in the beam path between the laser and the detector. The vacuum container is placed into the beam path and the absorption curve A.sub.M(.lamda.) of the gas is determined in the beam path into which the vacuum container has been placed and the absorption curve A.sub.R(.lamda.) is made to coincide by compression with the absorption curve A.sub.M(.lamda.A) outside of the absorption curve of the gas, the extent of compression being determined by a proportionality factor c, the following relation for determining the gas to be measured in the vacuum container being: A.sub.G(.lamda.)=A.sub.M(.lamda.)- cA.sub.R(.lamda.).