Cordierite Mg2Al4Si5O18 is a material for diesel particulate filters (DPF) with high potential. Its resistance to a simplified model ash has been tested on single crystals at temperatures up to 1050°C, which are realistic for use under ‘worst case’ conditions. A mixture of Ca, Mg and Zn phosphates, which are typical main constituents of many DPF ashes, was used as model ash. Single crystals were examined in order to investigate the orientation dependence of attack due to the strongly anisotropic nature of the Cordierite crystal lattice. Strong corrosion by the ash occurs at 1050 °C, connected to excessive ash melting. The molten ash attacks Cordierite by fast dissolution of the substrate with melt (super)saturation within minutes. Anisotropy of the dissolution process could not be detected. The initial kinetics are dominated by saturation effects, which slow down corrosion. The saturated melt attacks Cordierite by reaction processes leading to the formation of new crystalline phases. This process is much slower than the initial dissolution process but may significantly contribute to the destruction of Cordierite substrates if large contact areas between ash melt and Cordierite exist.