An ozone-based disinfecting device is provided comprising a mixer (2) having a generally hollow body with a water inlet for water under pressure, a spray nozzle (8) for generating a generally conical spray of water introduced by way of the water inlet, a contact chamber (7) communicating with a gas inlet (5) for ozone rich gases, and an outlet aperture (12) from the contact chamber. The outlet aperture is coaxial with the spray nozzle and axially spaced apart therefrom. The diameter of the outlet aperture corresponds substantially to the diameter of the conical spray at that position so that substantially no free space exists between the outside of the conical spray and the periphery of the outlet, in use. The contact chamber generally has a larger cross-sectional size than the diameter of the outlet aperture.
