Abstract: Agent and method for establishing different types of zone sealings particularly in oil wells and gas wells, wherein the zone to be sealed is supplied with a composition comprising: monomers, an initiator for heat-induced production of free radicals, a pot life extending inhibitor for stabilization of free radicals, and optionally other additives. The composition is subjected to radical polymerization induced by heat in the well zone and at a temperature of 5-120.degree. C., and establishes a cured mass, particularly at temperatures in the range of 65-100.degree. C. The composition is supplied with an at least partly unsaturated prepolymer selected from the group consisting of polyester and epoxy vinylester, including one or more vinyl-containing co-monomers selected from the group consisting of styrene, vinyltoluene, 2-hydroxyethyl-methacrylate and 2-hydroxypropyl-methacrylate.