The effects of cell geometry and dimensions on the thermal shock resistance of catalytic monoliths is examined analytically. Two cell geometries, namely square and equilateral triangle, are considered. Thermal gradients predicted by theory compare well with the experimental results. It is found that for equivalent thermal shock resistance the triangular cell requires lower coefficient of thermal expansion than the square cell. Also, as the cell density is increased for higher geometric surface area, both geometries require a reduction in thermal expansion coefficient to preserve their thermal shock resistance.The above comparison does not take into account some of the other considerations which affect the overall performance, such as manufacturing advantage and the conversion efficiency. Also, the triangular cell examined has a cell density of 236/in2 with 20% greater geometric surface area than the square cell with a cell density of 200/in2. If the dimensions of the square cell were adjusted so as to provide the same geometric surface area as the triangular cell, it would also require lower expansion coefficient although not as low as that required by the triangular cell.The laboratory thermal shock test to which the monoliths were subjected proved to be more severe than indicated by the threshold temperature measured by a thermocouple in the hot gas stream. Data from automobile manufacturers indicate that the honeycomb monoliths have more than adequate thermal shock resistance due to their satisfactory expansion characteristics.