Due to the need to properly quantify vehicle emissions in real world operation, Real Driving Emissions (RDE) test procedures will be used for measuring gaseous emissions on new EURO 6 vehicles.at the RDE 1 & 2: Commission Regulation (EU) 2016/427 of 10 March 2016 amending Regulation (EC) No 692/2008 as regards emissions from light passenger and commercial vehicles. Updated regulations have been enhanced to define RDE tests boundaries and data analysis procedures, in order to provide an accurate way to obtain representative results. The boundary conditions defined for vehicle testing include external atmospheric temperature, which can range from 0°C to around 30°C, for moderate conditions and -7°C up to 35°C for extended conditions in RDE tests. As a result of this range of possible test ambient temperature, pollutant emissions and energy consumption can vary considerably. Since the cold start phenomenon occurs in internal combustion engine (ICE) powered vehicles before the ICE reaches its most effective operation temperature, it affects both fuel consumption (due to higher heat losses) and pollutant emissions (mainly due to low exhaust temperature below activating temperature for after-treatment devices like SCR). This is an issue relevant in regular daily operations of a vehicle, which will also pose uncertainties in RDE tests. Consequently, this work studies the effect of external environmental temperature in RDE tests, focusing on the analysis of the cold-start period in energy consumption and NOx emissions. Two vehicles (one diesel and one gasoline) were monitored, covering external temperatures from circa 1°C to 17°C (considered ranges of 5°C up to 15°C) in Lisbon, Portugal. A Portable Emissions Measurement System (PEMS) was used to collect 1 Hz data on vehicle dynamics, road topography, engine data and exhaust gas composition. Data collected on both powertrain configurations is compared and analyzed using European Commission proposed method for RDE tests, as well as other methods to observe the impact of the cold-start phenomena in the normal day-to-day usage of vehicles. For the determination of cold-start periods, coolant temperature data was used as proxy to determine cold and normal operation. Results indicate that the initial ambient temperature have impacts in the duration of the cold-start period during a RDE test but not as relevant as expected, mostly at low temperatures compared to higher temperatures. NOx emissions can be around 30% higher in near 5°C temperatures for the diesel vehicle tested when compared with the other temperatures range. For the gasoline vehicle tested, the effect of the ambient outdoor temperature is not as significant as in diesel vehicles.