This paper addresses the disparity between liquid and vapor permeation through polymeric fuel line materials. The paper presents a methodology for assessing the vapor and fluid permeation rates of polyamide (PA) and polyvinyl-idene fluoride (PVDF) materials currently used in fuel delivery systems. The effect of single and multi component fuels on liquid and vapor permeation rates is studied. Permeation test results indicate a significant difference between liquid permeation rates and vapor permeation rates for these materials.The associated issue of polymer component extraction, during fuel and vapor exposure, is also addressed. Qualitative and quantitative data on precipitable extracts is presented.