Automobile time-resolved emissions of CO, CO2, HC, and NOx during engine and catalyst warm-up have been analyzed by fitting the emissions to the product of vehicle tractive power and a series of gaussian functions whose relative magnitudes were allowed to vary in time. From this analysis the emissions were discerned into four components : (1.) the emissions due to vehicle power demand, (2.) key-on emissions, (3.) a catalyst warm-up emissions function, and (4.) a fast idle emissions function. Both the emissions associated with the engine and the catalyst warm-up decline exponentially with time. Two additional characteristics (a.) emissions occurring during idling and (b.) emissions due to catalyst cooling during idle were observed, but not quantified. Also, a semi-empirical formula to approximate cold start emissions for light duty cars which includes the vehicle tractive power, time constants which define the emissions decrease in time, and the power demand characteristics has been developed. The data included thirteen light duty cars with model years ranging from 1995 through 2010. The driving cycles included the FTP, ST01, and LA92.