diphenylamines having the formula : ## str1 ## wherein r 1 , r 2 , r 3 and r 4 are the same or different and are hydrogen , alkyl with up to 4 c atoms or alkoxy with up to 4 c atoms , are prepared by catalytically dehydrogenating the corresponding wholly or partially hydrogenated diphenylamines . the catalyst used is a nickel / chromium catalyst containing manganese and / or aluminum and / or copper and / or sulphates of the alkali metals or alkaline earth metals .