The present invention relates to a pneumatic tire with a built-in sealant layer comprised of a depolymerized butyl rubber based sealant precursor. The butyl rubber of the built-in sealant precursor composition is depolymerized in situ within the tire with an activated free radical generating organoperoxide. The organoperoxide is activated with a 2,2,6,6-tetra alkyl piperidine hindered amine. Representative of such hindered amines are, for example, a poly[[6-[1,1,3,3,-tetramethylbutyl)amino]-s-triazine-2,4-diyl][2,2,6,6,-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)imino]] compound. The sealant layer, if desired, may be of a color which contrasts with black. For such purpose the sealant layer may contain a colorant of a non-black color to a substantial exclusion of carbon black. The butyl rubber-based sealant precursor is built into the tire as a layer to form a tire assembly wherein the butyl rubber-based composite portion of the sealant precursor layer is at least partially depolymerized by a said activated organoperoxide during a subsequent curing of the tire at an elevated temperature in a suitable mold to form the tire having the resultant built-in sealant layer. The sealant composition may additionally contain conventional clay, exfoliated intercalated clay platelets and/or calcium carbonate.