Internal combustion engine (ICE) cold-start is an issue that occurs either in conventional and hybrid powertrains before the ICE reaches its normal operation temperature, affecting both fuel consumption due to higher heat losses, and pollutant emissions due to low catalytic converter temperatures. The study of cold start emissions on conventional powertrains has been extensively addressed, although typically under laboratorial conditions, however studies addressing the impact of this phenomenon on hybrid powertrains is still reduced. Hybrid electric (HEV) and plug-in hybrid electric (PHEV) vehicles usually incorporate technologies to manage the battery and ICE power supply leading to ICE on/off operation under regular driving, which can result in a decrease on catalytic converter efficiency (due to cooling). This work suggest a new approach for the analysis of cold-start NOx emissions on conventional and hybrid powertrains, providing an insight on cold-start effects, namely in terms of emissions, under on-road conditions, by using a Portable Emission Measurement System (PEMS) to collect 1 Hz data on vehicle dynamics, road topography and exhaust gas composition. Data collected on different powertrain configurations is compared and analyzed in order to observe the impact of the cold-start phenomena in the normal day-to-day usage of vehicles. Since it was not possible to collect catalyst temperature, engine coolant temperature was used as proxy to define hot and cold running fuel use and emissions. Engine data, vehicle dynamics and exhaust gas composition were analyzed using the Vehicle Specific Power methodology to compare similar power conditions. Results indicate that powertrain configuration affect differently the NOx emission index (EI) over the power demand, however, on average, the ratio between cold and hot NOx EI is 1.61±0.20.