The thermochemical degradation resistance of a typical cordierite diesel particulate filter (DPF) material by synthetic ashes typical of those arising in practice has been investigated over the temperature range 950°C–1250°C. Differential thermal analyses and heat treatment of pressed pellets were used to characterize the melting/transformation behavior of ashes representative of typical diesel fuel (ash A) and those typical of when the catalysts ferrocene (ash FeA) and cerium carboxylate (ash CeA) were present in the fuel and to study the interaction chemistry between powdered cordierite and the ash compositions. Additional experiments involved the application of surface coverings of ash to DPF specimens. The results obtained showed that filter performance would not be compromised by ash liquefaction/sintering as long as temperatures did not exceed 900°C for ash A, 970°C for ash FeA and 1100°C for ash CeA. For long time periods, compared to the expected application durations, liquid phosphates dissolve cordierite leading to the formation of Zn and Fe aluminate spinels. Overall, the results clearly indicate that thermochemical degradation of cordierite by ashes under conditions representative of typical diesel engine systems is highly unlikely at temperatures of 1100°C and below.