The New Jersey Department of Transportation (NJDOT) is currently sponsoring a research study at Rowan University to develop strategies for reducing diesel emissions from mobile sources such as school buses and class 8 trucks. This paper presents the results of mobile school bus testing that has been performed to quantify the emission reduction capabilities of various aftertreatment devices. Particulate filters from Johnson Matthey and Lubrizol were tested along with a diesel oxidation catalyst (DOC) from Nett Technologies. Three school buses equipped with a 1997 7.3L International T444E, a 1997 7.6L International DT466E, and a 1996 Cummins 5.9L ISB series engine were instrumented and tested at the Aberdeen Test Center at the Aberdeen Proving Grounds in Maryland. Exhaust gas emission measurements were made using a Sensors Semtech-D to measure CO, CO2, NO2, NO, O2, and HC, along with a Sensors PM-300 to measure particulate matter (PM). In addition to the exhaust emissions measurements, operating parameters such as instantaneous vehicle speed, engine speed, percent load and fuel flow rate were acquired from the engine electronic control module (ECM) during testing. To ensure repeatability of testing under conditions that accurately reproduce actual school bus operating conditions, a newly developed composite school bus test cycle was used. Results from experiments showed that the Johnson Matthey CRT reduced PM and CO emissions from 48%-79% and HC by 88%-95%. NOX emissions were also reduced on the average of about 10%. The Nett Technologies DOC was capable of reducing the HC by 64% in the two International engines and 39% in the Cummins engine. CO was reduced using the Nett DOC by 74-85%. PM was reduced by over 20% per bin size and the corrected NOX by 6%. The Lubrizol Purifilter had the maximum reduction in PM for each engine and bin size. However, the Purifilter only provided minor CO reductions.