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The transient, one-dimensional monolith model previously developed for gasoline emission control applications has been extended to study converter warmup behavior in the exhaust from a variable-fuel vehicle (VFV) running on mixtures of methanol and gasoline by including the catalytic oxidation of methanol which involves the formation of stable gaseous formaldehyde as a reaction intermediate. The model calculations show that the aldehyde formation increases gradually at the early stages of converter lightoff (when methanol conversions are low), peaks at ∼50% methanol conversion, and then declines rapidly with a further increase in methanol conversion. Consequently, for all cases of practical interest, the total amount of aldehyde produced during the converter warmup period correlates well with the time to converter lightoff, with lower aldehyde emissions predicted at faster converter lightoff.