The accurate sampling and characterization of automotive particulate emissions is important to quantify any potential threat to human health. EPA has issued a draft recommended practice to measure the total weight of particulates emitted over the FTP, but there are many parameters that are uncontrolled or can be varied within the scope of the draft recommended practice. The parameters were separated into two categories: those affecting particulate formation and those affecting particulate sampling. The variables under the former category were identified as tailpipe conditioning and heat transfer, dilution ratio, and mixing rate. Test data on the effects of each of these variables on measured particulate emissions are presented and analyzed. The variables under the latter category were identified as the tunnel configuration, sample probe and sample line, and the type of filter used for sampling. Only the filter type was identified as having a significant effect on measured particulate emissions, and test data on filters is presented and analyzed.