An improved process for the relatively rapid thermal stabilization of a multifilamentary acrylic fibrous material in the substantial absence of filament coalescence is provided. The fibrous material is initially contacted with a solution of hydroxylamine (as described), and subsequently with a solution of ammonium permanganate, calcium permanganate, magnesium permanganate, or mixtures thereof (as described). The resulting fibrous material which possesses a chemically altered structure is heated in an oxygen-containing atmosphere until a thermally stabilized acrylic fibrous material is formed which is capable of undergoing carbonization and is non-burning when subjected to an ordinary match flame.