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Quenching of premixed flames at cold walls is investigated to study the importance of the model fuel choice for combustion modeling. Detailed chemical mechanisms for two different fuels, namely the low-molecular-weight fuel methane, and the more complex fuel iso-octane are employed. For both fuels the response of the flame to the very rapid heat loss at the cold wall is studied. The most important and significant difference between methane and iso-octane for this problem is the postquench oxidation of unburned hydrocarbons. Methane shows fast oxidation of unburned fuel and intermediate hydrocarbons whereas postquench oxidation for iso-octane is slow especially for the intermediate hydrocarbons. Furthermore, the Soret effect which is usually considered to be of minor importance appears to be important in modeling the rate limiting diffusion process. This is caused by different directions of the thermal diffusive transport for certain species.