This study presents real-time concentrations of traffic-related air pollutants during 499 trips conducted by 50 Parisian taxi drivers from PUF-TAXI project. Ultrafine particles (UFPs), black carbon (BC), and nitrogen dioxide (NO2)/carbon monoxide (CO) were measured inside vehicles by Diffusion Size Classifier Miniature(R), microAeth(R), and Gas-Pro(R), respectively, for nine hours. Vehicle/trip data characteristics were collected by questionnaires and on ambient conditions by monitoring stations. The associations between pollutant levels and their potential determinants were analyzed using generalized estimating equation model. Determinants of in-vehicle pollutants levels were identified: (1) ambient factors (meteorology and ambient pollution)-affecting BC, NO2, and CO; (2) vehicle characteristics-affecting all pollutants; and (3) trip-related driving habits-affecting UFP, BC, and CO. We highlight that commuters can, therefore, avoid high in-vehicle air pollutant concentrations mainly by (1) closing windows and activating air-conditioning under air recirculation mode in congested traffic; (2) smooth driving; and (3) maintaining cabin air filters.