The Bag Mini-diluter (BMD) is a proportional exhaust sampling system that is being studied as an improved measurement system for ultra-low level vehicle exhaust emissions. The traditional method for sampling vehicle exhaust has been the constant volume sampler (CVS) technique. This method dilutes the entire exhaust output from the vehicle, meters the mixture, and then takes a proportional sample for measurement. In contrast, the Mini-diluter sampling method meters a small sample of raw exhaust, and then dilutes this sample to a fixed dilution ratio. This approach offers new opportunities to improve the quality of the sample measurement at very low levels, which will be crucial for accurate vehicle exhaust emission measurements on vehicles that meet the ULEV and SULEV standards.A number of test programs have compared the performance of the Mini-diluter to the CVS on vehicles certified to Tier 1 and LEV standards, and the results demonstrated favorable correlation. The test program discussed in this paper uses the lessons learned from previous testing to focus on the measurement of hydrocarbon emissions at very low levels. Additionally, previous data demonstrated that the measurement of carbon dioxide was a concern due to the critical need for accuracy and repeatability in the fuel economy measurement. Numerous improvements were implemented on the Mini-diluter and exhaust flow measurement equipment to improve performance and to limit sampling equipment background emissions. The response time and accuracy of the Mini-diluter measurement system was improved. Moreover, new quality control techniques were used to study the performance of the Min-diluter and exhaust flow measurement systems because typical vehicle variability makes it difficult to detect the small differences between the Mini-diluter and the CVS. The test program concluded by evaluating ULEV vehicle emissions measurements on parallel Mini-diluter and CVS systems.