An inhaler comprising a reservoir (5) of inhalable composition an outlet valve (7) to control the flow of composition from the reservoir the valve outlet orifice (32) having a maximum dimension h measured in the direction of opening when fully opened an expansion chamber (33) downstream of the valve having a length L and diameter D measured half way along the expansion chamber and an exit orifice (31) at the downstream end of the expansion chamber the orifice having a length 1 and a diameter d wherein 0.1 < h/d < 1.0 0.05 < h/D < 0.25 1 < D/d < 10 5 < L/D < 15 0.1 < 1/d < 5.
