A diffusion device intended to accept a peripherally flanged capsule (11, 12) containing a product that is to be diffused into a stream of water, the device comprising: - a body (3, 4, 110, 303) intended to be fixed to a water circuit; - a bearing surface (47, 347) borne by the body directly or indirectly and forming a lower surface of a housing for a peripherally flanged capsule containing a water-soluble product, - a passage (16, 316) for letting the product out of the capsule housing as far as an outlet (21, 321) of the passage; - a knob (334, 34) mounted with the ability to rotate on the body (3, 4, 110, 303), the knob comprising an upper part projecting in the direction of the axis of rotation (25, 325) of the knob beyond the bearing surface (47, 347), characterized by - the knob and the bearing surface being mounted with the capacity for helicoidal rotation one with respect to the other; - the knob (34, 334) being able to be brought, in a closing movement with respect to the body, from a position in which the device is open, in which position the housing offers an opening of a dimension greater than a diameter of the flange of the capsule and intended to allow the flanged capsule to be inserted into the housing facing the bearing surface (47, 347); - the closure movement of the knob being able to bring the lower face of a rim (82, 83, 382, 383) of the device to face the bearing surface (47, 347), the closure movement of the knob comprising a rotational movement of the knob with respect to the body causing a helicoidal movement between the knob and the bearing surface capable of bringing a lower face of the rim (82, 83, 382, 383) and the bearing surface (47, 347) closer together in order by compression to hold a peripheral portion of the flange of the capsule between the rim and the bearing surface in a sealed manner when the device is in a closed position. - a body (3, 4, 110, 303) intended to be fixed to a water circuit; - a bearing surface (47, 347) borne by the body directly or indirectly and forming a lower surface of a housing for a peripherally flanged capsule containing a water-soluble product, - a passage (16, 316) for letting the product out of the capsule housing as far as an outlet (21, 321) of the passage; - a knob (334, 34) mounted with the ability to rotate on the body (3, 4, 110, 303), the knob comprising an upper part projecting in the direction of the axis of rotation (25, 325) of the knob beyond the bearing surface (47, 347), characterized by - the knob and the bearing surface being mounted with the capacity for helicoidal rotation one with respect to the other; - the knob (34, 334) being able to be brought, in a closing movement with respect to the body, from a position in which the device is open, in which position the housing offers an opening of a dimension greater than a diameter of the flange of the capsule and intended to allow the flanged capsule to be inserted into the housing facing the bearing surface (47, 347); - the closure movement of the knob being able to bring the lower face of a rim (82, 83, 382, 383) of the device to face the bearing surface (47, 347), the closure movement of the knob comprising a rotational movement of the knob with respect to the body causing a helicoidal movement between the knob and the bearing surface capable of bringing a lower face of the rim (82, 83, 382, 383) and the bearing surface (47, 347) closer together in order by compression to hold a peripheral portion of the flange of the capsule between the rim and the bearing surface in a sealed manner when the device is in a closed position.