The present invention provides a process for working up alkaline waste water which is formed during washing of crude nitrobenzene obtained by nitration of benzene, wherein(i) the alkaline waste water is heated to a temperature of from 150° C. to 500° C. under an increased pressure with respect to atmospheric pressure with exclusion of oxygen;(ii) a base is added to the waste water obtained in (i); and(iii) the waste water obtained in (ii) is purified further by stripping with a stripping gas and the stripping gas strewn loaded with impurities is then cooled to a temperature of from 10° C. to 60° C.