Stringent emissions standards for HC, CO and NOx have necessitated the development of thin wall ceramic substrates which offer higher surface area, larger open frontal area and lower thermal mass. Such substrates offer the additional benefit of being compact which make them ideal for manifold mounting in the engine compartment. These attributes of ceramic substrates, following washcoat and catalyst application, translate directly into quick light-off, high conversion efficiency and low back pressure. To preserve these advantages at high operating temperature and still meet 100,000 mile vehicle durability, the thermomechanical interaction between the substrate and thin wall washcoat system must be managed carefully via formulation, % loading and the calcination process.This paper presents the physical properties data for thin wall ceramic substrates before and after the washcoat application. The impact of washcoat adhesion and % loading on these properties over the operating temperature range will also be discussed. These data in turn will help assess the long-term durability of thin wall ceramic converter system.