A fast light off flow-through ceramic substrate is provided that is particularly adapted for use as a catalytic converter. The substrate is formed from a body of ceramic material having axially opposing inlet and outlet ends for receiving and expelling the flow of automotive exhaust gas, respectively. The body contains a network of walls coated with a catalyst that define axially-oriented flow-through cell channels. The average thermal mass (ATM1) of a first axial region of the walls adjacent to the inlet end is at least 20% less than the average thermal mass (ATMTOT) of all of the walls. The lower average thermal mass of the walls in the first region advantageously shortens the light off time for the catalyst within the substrate to effectively neutralize automotive pollutants. The reduction of the average thermal mass in only the first axial region of the walls advantageously maintains the strength of the resulting body of ceramic material, and further increases the cool down time of the body.