A gas diffusion arrangement (22) for a gas turbine engine (10) is disclosed. The gas diffusion arrangement (22) has an inlet (26) and an outlet (28) for the gas, and comprises diffusion means (32) having an upstream region (38) and a downstream region (40). Distribution means (41) is arranged between the downstream region (40) and the outlet (28) to distribute the gas into a desired flow pattern. The area ratio of the cross-sectional area of the downstream region (40) to the cross-sectional area of the upstream region (38) is greater than 1.5 to 1.