Abstract: Described are photopolymerizable compositions containingA. an ethylenically unsaturated compound,B. about 3-95% by weight of an organic polymeric binder,C. about 0.1-5% by weight of a nitroso dimer which is a noninhibitor of free-radical polymerization but thermally dissociates to nitroso monomer which is an inhibitor of free-radical polymerization, andD. about 0.1-2% by weight of an organic, radiation-sensitive, free-radical generating system.Positive-working, contour images are formed by applying a layer of this composition to a substrate, imagewise exposing the photopolymerizable layer to actinic radiation through an image-bearing transparency at about 20.degree.-65.degree. C, whereby free-radicals are consumed by nitroso monomer, deactivating the nitroso dimer inhibitor system, reexposing at least the unexposed portion of the photopolymerizable layer to actinic radiation while continuing to maintain the nitroso dimer inhibitor system in the deactivated state, and developing the resulting image.