Piperidine derivatives having the formula ##SPC1##WhereinR' represents an alkyl group, a substituted alkyl group, an acyl group, an alkoxycarbonyl group, a substituted alkoxycarbonyl group, an amino group, a substituted amino group, or nitroso group;X represents oxygen atom or sulfur atom;Y represents oxygen atom, sulfur atom or a group of the formula = N -- R" in which R" is hydrogen atom, an alkyl group or a substituted alkyl group;Z represents oxygen atom or a group of the formula >N -- R'" in which R'" is hydrogen atom, an alkyl group or a substituted alkyl group;n is an integer of 1 through 4 inclusive; andR represents, when n is 1, an alkyl group, a substituted alkyl group, an aryl group, a substituted aryl group, a cycloalkyl group, an alkoxycarbonyl group, a substituted alkoxycarbonyl group, a substituted phosphino group or a substituted phosphinyl group, when n is 2, an alkylene group, an alkenylene group, an arylene group, a substituted arylene group, an aralkylene group; an alkylenediphenylene group, a bis-(acyloxyalkylene) group, an alkylene-bis-(oxycarbonylalkyl) group, a dialkylene ether group or a diphenylene ether group, when n is 3, an alkanetriyl group, a tris(acyloxyalkylene) group, an alkane-tris(oxycarbonylakyl) group or a group of the group ##SPC2##In whichp is an integer of 1 through 8 inclusive, and, when n is 4, an alkanetetrayl group, a tetrakis-(acyloxyalkylene) group or an alkane-tetrakis-(oxycarbonylalkyl) group.The piperidine derivatives (I) of this invention are prepared in various manners and useful as stabilizers for synthetic polymers against thermal- and photo-deterioration thereof.