A molecular sieve type gas separation apparatus (10) for separating product gas from a gas supply (14), the apparatus (10) comprising at least one sieve bed (11-13) which during an adsorption charge phase of a cycle, adsorbs non product gas in the gas supply (14), and in a desorption regeneration phase of the cycle desorbs the previously adsorbed non product gas, the apparatus further comprising outlet means (18) to which product gas is delivered during the adsorption charge phase, and outlet means (31) for non product gas during the desorption regeneration phase, sensor means (36) responsive to the concentration of a constituent in the product gas, and control means (20) to switch the apparatus (10) between the adsorption charge and desorption regeneration phases, characterised in that the control means (20) is adapted to adjust the duration of adsorption charge phase relative to the duration of the desorption regeneration phase in each cycle.