A one-part, air-activatable, polymerizable composition comprising:(a) at least one free-radically polymerizable monomer, and(b) an activator system for effective polymerization of the free-radically polymerizable monomer, said activator system comprising at least one auto-oxidizable compound of a formula which includes the structure I: ##STR1## where x is 0 or 1, and where the lines representing unfilled valencies indicate bonds to carbon, hydrogen or hetero atoms, alone or in combination with a weak acid;with the proviso that:when x=0, the nitrogen atom in structure I is not bonded to a second nitrogen atom;and when x=1 there is a structure I in the compound in which the >C.dbd.C>moiety does not form part of a phenyl ring;with the proviso that the composition does not contain a peroxide, or a peroxide precursor which produces peroxide in the absence of air or any ingredient which is a significant source of radicals in the absence of air.Preferred auto-oxidizable compounds are selected from the group consisting of partially hydrogenated pyridines, condensation products of cyclic ketones and ureas, Schiff's bases, indoles, pyrroles, imidazoles, piperazines, carbazoles, tetrahydroquinolines, and substituted derivatives thereof, particularly dihydropyridines.The composition may also contain a soluble ionic salt, particularly a cobalt or iron salt.The compositions as described are stable on storage in the absence of air, are activated by exposure to air, and will then cure in either the presence or absence of air.