A method is described for recycling hydrogen (H2) supplied to a chamber (10) in a gas stream comprising hydrogen and at least one other gas, such as silane. A gas comprising at least hydrogen is drawn from the chamber (10) using a first vacuum pump (32) that exhausts gas therefrom at a sub-atmospheric pressure. A portion of the gas exhausted from the first vacuum pump (32), for example between 70 and 95% of this gas, is diverted away from a second vacuum pump (34) backing the first vacuum pump (32). In one embodiment, the diverted portion of the sub-atmospheric pressure gas is treated to produce a purified gas comprising hydrogen, which is stored in a storage vessel (14). The composition of the purified gas is analysed, and, depending on the results of the analysis, at least one of hydrogen and silane is added to the stored gas so that the composition of the stored gas is similar to that of the gas initially supplied to the chamber (10). Gas is then supplied to the chamber (10) from the storage vessel (14).