Regulatory agencies are in the process of implementing an in-use testing program for heavy-duty diesel vehicles that will include testing with portable emissions measurement systems (PEMS) under in-use driving conditions. An important aspect of this regulation is the Measurement Allowance program where EPA, CARB, and the Engine Manufacturers Association (EMA) are working together to systematically evaluate various sources of error for gaseous and PM measurements with PEMS in comparison with laboratory measurements. This error is then accounted for in the regulatory standards as a “Measurement Allowance”. A comprehensive program has already been conducted for the gas-phase measurement allowance, with the PM measurement allowance program about to begin. The main objective of this work was to provide preliminary measurements from PM PEMS to assess the accuracy of PM measurements under in-use conditions and provide a basis for the development of the more comprehensive Measurement Allowance program. The MASC utilized the University of California, Riverside (UCR) Bourns College of Engineering – Center for Environmental Research and Technology’s (CE-CERT) Mobile Emissions Laboratory (MEL) to perform the initial in-use PM PEMS evaluation. For this program, PM PEMS were directly compared with the MEL over a series of different onroad driving conditions. Prior to the on-road testing, MEL underwent a 40CFR Part 1065 selfaudit focused on PM sampling. In-use measurements were made from three different Class 8 tractors representing three different engine manufactures. One truck had a 2000 Caterpillar engine without a DPF and the other two were equipped with OEM DPFs, one from Cummins and the other from Volvo. Each of the 2007 vehicles was modified to vary their emission levels using regeneration, ECM recalibrations, and a DPF bypass. The on-road driving courses included segments near sea level, in coastal regions, in desert regions, and on longer uphill inclines. The goal was to test the vehicle at or slightly above the Not-To-Exceed (NTE) threshold to investigate sources of error for the PM instruments at levels where their performance is most critical. The bsPM level varied from 0.1 g/hp-h to 0.0003 g/hp-h over the different vehicles and operating conditions where one vehicle had high EC, one had high OC, and another had a substantial amount of sulfate. In addition to varying composition and bsPM level, one of the vehicles showed a significant reduction in particle size thus challenging the PM PEMS measurement systems. PM measurements in real-time were made with a variety of different PM instruments from manufacturers preparing for the PM Measurement Allowance program, including a Horiba OBSTRPM system, a Sensors SemtechDS PPMD (QCM), and an AVL Photoacoustic MicroSoot Sensor, as well as other commercially available instruments such as a Dekati DMM and TSI Dustrak. These measurements were directly compared with gravimetric PM mass measurements that were collected with the MEL under 1065 compliant sampling conditions. Measurements were made under conditions where NTE events would be expected (e.g., uphill driving segments) and for varying durations to provide a range of mass loadings. The results of this study are expected to be an important component of PM Measurement Allowance program development.