The substrate is used in a catalytic converter can to which a cylindrical inlet pipe leads and has an inner catalytic zone portion, an outer catalytic zone portion and an insulation material thermally separating the portions. The insulation extends through the substrate and has a uniform cross-section substantially defined by the intersection of two notional cylinders and the upstream face of the substrate, each notional cylinder having: a nominal diameter that is between 1.08 and 1.20 of the diameter of the inlet pipe; a thickness of 1-4 mm; and an axis aligned with the gas direction at the point of maximum velocity at the intersection of the inlet pipe and the can. One of the cylinders is associated with the gas flow at the lower limit of the operating range and the other of the cylinders is associated with the gas flow at the upper limit of the operating range.