This paper presents an investigation into an aspect of how plug-in hybrid electric vehicles (PHEVs) could impact the electric power system. The investigation is focused on the impact of the additional electrical load PHEV charging will have on primary energy source utilization and subsequent environmental air pollution (EAP) as emissions are transferred from vehicle tailpipes to powerplants. A methodology is presented for evaluating the impact on primary energy source utilization, considering all the operating constraints of an electric power system, as well as, the realistic operation of PHEVs. Examples of energy source utilization impacts are presented for various levels of PHEV penetration on a specific power system. In general, PHEVs cause a shift of fuel utilization from gasoline to, a more diversified fuel source, utilized by electric power utilities. The results are particularly sensitive to the generation mix of the specific power system simulated.