This invention relates to N-acylamino aryl derivatives of the general formula (I), wherein R 1 is halogen, halogen-(C 1 -C 6 )-alkyl, cyano, C 1 -C 6 -alkoxy or halogen-(C 1 -C 6 )-alkoxy; R 21 , R 22 , R 23 and R 24 independently from each other are selected from the group consisting of hydrogen, (C 1 -C 6 )-alkyl, halogen, halogen-(C 1 -C 6 )-alkyl, hydroxy, C 1 -C 6 -alkoxy or -CHO; R 3 is hydrogen or C 1 -C 3 -alkyl; R 4 , R 5 independently from each other are selected from the group consisting of hydrogen, C 1- C 6 -alkyl, C 1 -C 6 -alkoxy or -COO(C 1 -C 6 )alkyl; or R 4 and R 5 form together with the C-atom to which they are attach a C 3 -C 7 -cycloalkyl ring; R 6 is -CO-NR 7 R 8 ; -COO(C 1 -C 6 )-alkyl, -CN, -NR 2 or -NHC(O)R; R 7 and R 8 independently from each other are selected from the group consisting of hydrogen, C 1- C 6 -alkyl, NH 2 or hydroxy; R is hydrogen or C 1 -C 6 -alkyl; n is 0, 1, 2 or 3. X is -CHRO, -OCHR, -CH 2 S-, -SCH 2 -, -CH 2 CH 2 -, -CH=CH- or -C≡C-; and to pharmaceutically active acid addition salts thereof. It has been found that the compounds of general formula (I) are selective monoamine oxidase B inhibitors and they are therefore useful in the treatment of diseases mediated by monoamine oxidase B inhibitors, for example for the treatment of Alzheimer’s disease or senile dementia.