diaryl sulphides and diaryl selenides of formula : ar -- x -- ar where x = s or se and ar = aryl are made by the action of sulphur and sulphur dioxide or selenium and selenium dioxide on a compound of formula : ar -- h the process is especially useful for the preparation of phenothiazines of formula ## str1 ## in which x = s or se and z and z 1 , which may be identical or different , are each hydrogen , halogen , alkyl or alkoxy .