Hybrid electric vehicles (HEVs) can potentially reduce vehicle fuel consumption and CO2 emissions by using recuperated kinetic vehicle energy stored as electric energy in a hybrid system battery (HSB). Low ambient temperatures can affect the overall HEV powertrain operation under warm-up and hot driving conditions and, consequently, affect fuel consumption and emission performance. The present study investigates the influence of low ambient temperatures on HEV fuel consumption and pollutant and CO2 emissions for five in-use HEV models. Chassis dynamometer measurements have been conducted at different set ambient temperatures using a real-world driving cycle suitable for investigating vehicle cold-start emissions. The main observation is that the amount of HEV cold-start extra emissions (CSEEs) of regulated pollutants are reduced by 30% to 85% on average in comparison to sample CSEEs of conventional gasoline vehicles. The results for HEV CSEEs of CO2 and fuel consumption are mainly similar than those of conventional gasoline vehicles except for CSEEs of some HEVs at the ambient temperature of 23 degrees C. There, increased CSEEs are observed that exceed maximum sample CSEEs of conventional gasoline vehicles, reaching values for CO2 between 155 [g/start] and 300 [g/start] even though the test runs were initiated with maximum initial state of charge (SOC) of the HSB. Because SOC of the HSB considerably influences the fuel consumption of HEVs, this aspect should be further investigated in regard to the effect of low ambient temperatures on HEV fuel consumption and CO2 emissions. Moreover, no particular influence of low ambient temperatures on HSB performance was observed during hot-phase operation. (C) 2012 Elsevier Ltd. All rights reserved.