Intumescent vermiculite flakes useful in mountings of catalytic converters are prepared by partially dehydrating vermiculite ore to afford a uniform content of chemically bound water from about 1.0 to 3.2% by weight and a uniform bulk density from about 0.2 to 0.9 g/cm.sup.3. Such a mounting affords good support to the monolith at warm-up temperatures, exerts adequate holding forces at operating temperatures without danger of cracking the monolith, and does not incur gradual reduction in holding forces at operating temperatures. The mounting layer can be applied as either a mat or a paste.