The invention relates to a device for irradiating a liquid with electromagnetic radiation, in particular for sterilizing an in particular flowing liquid by means of UV radiation (UV reactor), comprising a container having an inlet for receiving the liquid and having an outlet for releasing the liquid from the container, wherein within the container a variable or adjustable irradiation zone is provided for irradiating the liquid with electromagnetic radiation, in particular UV radiation, emitted by a radiation source. In the irradiation zone the liquid is configured in the form of a liquid layer having the layer thickness which extends between the underside of the container and a gas bubble expanding above the liquid layer.