The recombination rate coefficient of the ISX‐B vacuum vessel wall has been measured by observing the rate of decrease of gas pressure in the vesssel during a glow discharge. A simple analytic expression which relates the rate of change of the pressure to the recombination rate is derived and verified by comparison with a complete numerical simulation. Comparison of the recombination coefficient measured in ISX‐B with previous laboratory measurements shows that the measured value is typical for a stainless steel surface covered with an oxide layer.