It is a well-known phenomenon that the bulk of hydrocarbon emissions emitted from internal combustion engines occurs during the coldstart period, thus necessitating new control strategies to reduce hydrocarbon emissions. A simplified model of the catalytic converter during coldstart is developed, employing thermal dynamics, oxygen storage and static conversion curves. It is subsequently shown that oxygen storage does not play a significant role during coldstart. An empirical parametrization of the catalyst efficiency using Wiebe profiles is proposed to capture general conversion trends of the catalyst. Finally it is shown that the ability of the catalyst to reduce hydrocarbon emissions during coldstart is dominated by its thermal behavior and ability to rapidly achieve lightoff. Model predictions are compared to experimental results for catalyst warmup with good agreement.