An inerting system for an aircraft fuel tank (10) uses an air separation device (12) to produce a supply of nitrogen-enriched air to replenish the fuel tank on consumption of fuel and to maintain the pressure difference across the walls of the tank (10) below design limits. The air separation device (12) provides substantially the whole of the mass of the gas required and so there is substantially no inward venting. This provides a substantially homogenous nitrogen content in the air in the tank (10).