Abstract: Improvements in oxygen generators and inert gas generators in aircraft. Separate gas generators are inefficient and thus present the problems of excess weight and heavy space and bleed air requirements. This invention provides an integrated system of generating oxygen and inert gas to pre-enrich the supply gas and increase overall efficiency. Bleed air is supplied to a first module (14) that separates it into a nitrogen enriched product gas and an oxygen enriched waste gas. The waste gas is fed into a second module (26) which separates it into an oxygen enriched product gas and a second waste gas. The modules (14, 26) may be reversed, with bleed air being fed into the oxygen generator and waste gas being fed into the inert gas generator. The system may include three modules (34, 42, 56). The waste gas from first module (34) is fed into second module (56), and the waste gas from second module (56) and the product gas from first module (34) are fed into third module (42).