A device for mixing and cooling two reactive liquids, comprising a bundle type heat exchanger with parallel tubes, a head space open to one end of all tubes, a first inlet to the head space for introducing a first liquid and a second inlet to the head space with a multitude of nozzles for introducing a second liquid, the nozzles being located within the head space and oriented to direct the introduced liquid transverse to the axis of the tubes of the tube bundle, is useful for making peroxomonosulphuric acid from 85 to 98% by weight sulphuric acid introduced into the first inlet of the device and 50 to 80% by weight aqueous hydrogen peroxide introduced into the second inlet of the device.