The physicochemical and toxicological characteristics of the volatile and non-volatile fractions of particles emitted from different engines, fuels, and emissions control were studied. The test fleet comprised of four heavy-duty diesel vehicles, a Kenworth truck served as a "baseline vehicle". Different emission control technologies, i.e., a Continuously Regenerating Technology [CRT®], consisting of a diesel oxidation catalyst (DOC) followed by an un-catalyzed trap, and CRT® in combination with a SCR system (zeolite or vanadium based SCRT). Most of the control devices reduced the particulate mass significantly while increasing the particle number concentration. Nucleation mode particles were highly volatile in nature and the predominant chemical species present in the vehicles' exhausts were sulfate and total carbon. This is an abstract of a paper presented at the 101st AWMA Annual Conference and Exhibition (Portland, OR 6/24-27/2008).