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Increasing interest in gasoline engine emissions has focused attention on the fuel compositional and emissions effects that govern NOx conversion over the catalyst. This study reports the transient effects of individual species emissions and catalyst conversions on NOx conversion made using Fourier Transform Infra Red (FTIR) spectroscopy of the engine-out and tailpipe emissions (regulated and speciated) during the testing of a catalyst equipped gasoline vehicle run on multi-component model fuels over the standard European cycle. FTIR measurements confirm that transient NO conversion is directly correlated with that of CH4, especially within the Urban Drive Cycle (EUDC). Other hydrocarbon species do not govern the transient variability in NO conversion. This vehicle maintained ϕ≤ 1.0 practically throughout the EUDC and consequently no correlation was seen between transient NO conversion and equivalence ratio.