The absolute cold-start extra emission (in gram) is described as the additional emission level obtained under cold conditions compared to the emission value measured on the same driving distance or time period under hot conditions (Joumard et al. 2007). In order to define cold-start additional emissions and the cold-start transient durations, measurements performed on repetitive real-world driving cycles have to be performed, as defined by Weilenmann (2005). However, it is not simple to apply this methodology to motorcycles, as no repetitive driving cycles have been developed for this vehicle category, and only little research on this aspect has been conducted in the last years. In this section, a calculation procedure is shown to model the cold-start transient of motorcycles. Cold-start extra emissions during the transient duration of the engine can be characterized by a cold instantaneous emission factor fcold (t) (Iodice et al. 2011), expressed as mass per time unit