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.