Particulate emissions from heavy-duty buses were measured in real time under conditions encountered during the standard Central Business District (CBD) driving cycle. The buses tested were equipped with 1994 Detroit Diesel Engine Corporation 6V92-TA engines, and some included after treatment devices on the exhaust. Instantaneous, time-resolved measurements of CO2 and amorphous carbon concentrations were obtained using an optical extinction technique and compared to simultaneous results obtained using conventional dilution tunnel sampling methods. Good agreement was obtained between the real-time extinction measurements and the diluted CO2 and cycle-integrated filter measurements. The instantaneous measurements revealed that acceleration transients accounted for roughly 80% of the particulate mass emitted during the cycle but only about 45% of the fuel consumption. More detailed examination of the acceleration periods revealed that in fact, fuel injection transients created the majority of the soot not simply high load conditions. Sometimes particulates were stored in the exhaust systems for a short time (on the order of one second), which delayed their emissions. Hence, tighter control of fueling rates during transients may significantly reduce vehicle particulate emissions without reduction in peak torque. Finally, the time-resolved measurements of particulate emissions from a bus equipped with a particulate trap demonstrated the need for transient warm-up procedures rather than extended steady-state warm-up intervals prior to the conduction of dilution tunnel tests.