In the next years, the number of circulating plug-in hybrid electric vehicles was expected to increase worldwide. Hybrid electric/gasoline powertrain is considered a promising solution to meet the future goals of greenhouse gas emission reduction by road transport sector. This paper investigates the emissive and energetic behaviour of a plug-in hybrid electric passenger car over on road testing. Measurements of CO, CO 2 , NO x exhaust emissions and of main electric motor parameters were carried out by using portable emission measurement system over urban, rural and motorway routes in Naples (Italy). Routes were selected for complying with European Real Driving Emission (RDE) legislation. Moreover, an additional urban route, characterised by strong road grade and non- RDE compliant, was investigated. RDE test allowed to assess the influence of the battery state of charge, hybrid mode (charge sustaining and charge depleting), road grade and vehicle air conditioning on the exhaust emission and energy consumption. An overall advantage of charged battery and charge depleting mode was observed in terms of CO 2 emissions and fuel consumption. CO and NO x emissions are strongly influenced by driving when the vehicle's internal combustion engine starts. When it occurs in motorway patterns, CO and NO x peak emissions are observed at tailpipe. Pure electric driving, with consequent zero emissions, are guaranteed over urban strong uphill when battery is full charged.