Abstract: The invention relates to a process of using certain piperidine compounds of formula I ##STR1## in which R.sub.1 is hydrogen, hydroxyl, lower alkyl, lower alkoxy or acyl,R.sub.2 is a substituted or unsubstituted mono- or bicyclic radical which is aromatic in type,R.sub.3 is oxygen, --NH-- or --N(C.sub.1-4 -alkyl)-- andR.sub.8 independently at each occurrence is hydrogen or methylfor stabilizing natural or synthetic, organic or inorganic pigments or pigment mixtures in natural or synthetic, polymeric or prepolymeric substrates against the adverse effects of heat and/or light on the light fastness and color stability, respectively, especially against alterations in shade or light-induced bleaching. The invention also relates to a masterbatch composition.

Abstract: Disclosed is a process for stabilizing polyamides against the damage effected by light, heat and/or oxidation while improving the differential dyeability, and processing stability therefore, comprising incorporating therein by melt-processing a synergistic additive system resulting in a uniform and intimate mixture with said polyamide, said additive system is selected from the group consisting of (I), (II) and (III)(I) is from 0.01% to 1% by weight of a sterically hindered phenylphosphonite (Ia) with from 0.05% to 5% by weight of oxanilide (Ib) or said (Ia) together with from 0.05% to 5% by weight of an aromatic di- or tri-carbonyl compound containing at least one, and preferably two hindered amine moieties (Ic);(II) is from 0.05% to 5% by weight of oxanilide (IIa) together with from 0.05% to 5% by weight of a aromatic di- or tri-carbonyl compound (IIb) containing at least one hindered amine moieties;(III) is from 0.01% to 1% by weight of a sterically hindered phenyl phosphonite (Ia), from 0.