It is manifest in Table4(above all observing the cold/ hot emission quotients for pollutant and vehicles) that a substantial difference exists between emission during the cold-start and the hot phases of the tested Euro-3 motorcycles, as effect of improved engines in combination with catalytic technology used on these vehicles. CO formation in engine, as known, depends mainly on air/fuel mixture equivalence ratio; as regards all the tested vehicles, mixture enrichment during cold-start (outside the optimal catalyst range) increases these emissions in comparison with the levels recorded for steady engine operation. Besides, at low ambient temperatures, the cold engine contrasts fuel vaporization, thus raising the formation of unburned fuel and then leading to increased HC emissions. During the cold-start, therefore, the engine and catalytic converter are not at their ideal functioning conditions; assuming the rich value of the air–fuel mixture and the catalytic converter that fails to reach the light-off temperature, these highperformance motorcycles produce higher cold start emissions of CO and unburned hydrocarbons.