N,N-Diaryl-ureas of the formula R.sup.1 --NR.sup.2 --CO--NH--R.sup.3 wherein R.sup.1 and R.sup.2 denote aryl and R.sup.3 denotes alkyl, aralkyl or aryl, can be prepared by a process in which a diarylamine of the formula R.sup.1 --NH--R.sup.2 is reacted with an isocyanate of the formula R.sup.3 --NCO wherein R.sup.1, R.sup.2 and R.sup.34 have the above meaning, in the presence of acid compounds and in the presence or absence of an inert solvent and/or diluent at elevated temperature.