Abstract A significant proportion of anthropogenic air pollution can be attributed to the light-duty vehicle fleet. As new vehicle technologies are introduced to address fuel consumption and pollution concerns, methods for estimating emissions from alternative vehicles will become more pertinent. Primary mechanisms of ultrafine particle generation from light-duty vehicles are nucleation or condensation of gas-phase pollutants following gasoline combustion and exhaust after treatment. Previous studies have linked gas-phase pollutants to ultrafine particle number or mass emissions, but little work has been done to date to predict particle number emissions with vehicle operation parameters and gas-phase co-pollutants. Tailpipe emissions data from hybrid and conventional 2010 Toyota Camry vehicles were collected during real world driving over a period of 18 months. Samples from the tailpipe were transferred into the vehicles and analyzed in real-time by a TSI Engine Exhaust Particle Sizer and a MKS MultiGas 2030 Analyzer. The relationship of vehicle operating modes, gas-phase pollutants, and particle size distributions were investigated. Emissions data were binned by vehicle specific power (VSP) and three speed regimes (<25, 25-50, and >50 miles per hour) in accordance with current modeling methods. Initial results revealed the need for unique modeling approaches depending on vehicle type. Increasing VSP and speed operating modes generally corresponded to higher mean emission rates for the conventional vehicle. A shift of high emission events for the hybrid vehicle to the high VSP, mid-range speed regime was observed for gas-phase pollutants (i.e. carbon monoxide and ammonia) and particle number emissions. Additionally, particle emission rates were significantly higher for high VSP, low speed hybrid operation as compared to the conventional vehicle. The shift of high emission events to lower speed regimes were likely attributable to the transition between all-electric and engine-on hybrid operation as well as target engine loading during highway speed operation.