This test method described in this document covers a procedure to speciate that is@ to determine the amounts of each different fuel constituent that permeates across sheets@ films or slabs of plastic materials. One side of the sheet is meant to be in contact with either a liquid test fuel or a saturated test fuel vapor@ the other side is meant to be exposed to an environment free of fuel. The test fuel can either be a mixture of a small (usually smaller than ten) number of hydrocarbon@ alcohol and ether constituents or it can be a sample of a real automotive fuel@ e.g.@ one that may contain hundreds of different constituents. Furthermore@ Appendix A contains guidelines to speciate evaporative emissions from finished fuel system components such as fuel lines@ fuel filler pipes@ fuel sender units@ connectors and valves.