Abstract: This invention concerns oxygen dilution apparatus of the kind used when providing inhalation therapy, and a method of adjusting the air dilution of an oxygen supply for an inhalation mask. The invention involves a mixing valve (1) connected into the oxygen line supplying a face-mask. The valve body comprises an orifice (14) communicating with the atmosphere. The ratio of air to oxygen delivered to the mask is controlled by operating the valve to change the cross sectional area of the oxygen inlet port. Preferably this is done by rotating two parts (2,20) of the valve with respect to each other, which action causes the insertion or withdrawal of a stem (23) of varying cross-section in an aperture (19) of constant cross-section. The invention has the advantage that oxygen dilution is controlled by a geared-down motion which allows the two parts of the valve body to be moved in a reliable and foolproof manner between positions which result in different proportions of oxygen and air being delivered.