In a method and an apparatus for cleaning tar-bearing waste water (17), a mixture of water and hydrocarbons, e.g. comprising polyaromatic hydrocarbons and phenols, the mixture is separated into a low-boiling-point part and a high-boiling-point part, bringing the low-boiling-point part on vapour form in a boiler (1), and the low-boiling-point part is cracked in vapour form at a high temperature in a reactor (2), providing light combustible gases, which can be utilised in e.g. gas engines, gas turbines or the like. Furthermore, the high-boiling-point part may be used for energy supply to the process or other processes or as an alternative be cracked for providing light combustible gases.