The invention concerns 4-acylamino piperidin-N-oxyls of the formula: A<1>B<1> wherein A<1> stands for hydrogen or an organic group and B<1> stands for a group of formula (IIa) in which the substituents have the following meanings: R<1>-R<4> are C1-C4 alkyl (R<1> and R<2> and R<3> and R<4> can be bound to a 5- or 6-member ring); R<5> is H or C1-C4 alkyl; and R<6> is H or C1-C18 alkyl. The compounds in question serve to stabilise organic substances against the harmful effects of radicals, especially in the distillation of radically polymerising monomers such as styrene.
