Intumescent sheet material

A flexible intumescent sheet useful for mounting automotive catalytic converter monoliths comprising from 40% to 65% by weight of unexpanded vermiculite flakes having particle sizes of from about 0.1 mm. to about 6 mm. treated by extended soaking in an aqueous ammonium solution and being substantially completely ion-exchanged with NH.sub.4 + cations, from 25% to 50% by weight of inorganic fibrous material and from 5% to 15% of binder, said sheet upon exposure to heat from an engine exhaust being capable of undergoing thermal expansion, said sheet having a maximum percent negative expansion of about 10% at about 300.degree. C. and returning to its original starting thickness or greater at about 350.degree. C.