The outermost at. layer of perfluorinated thiol monolayers on gold and poly(tetrafluoroethylene) (PTFE) is analyzed by low-energy ion scattering. Abs. quantification of fluorine d. in this layer was achieved after calibrating the fluorine signal with a freshly cleaved LiF(100) single crystal. The fluorine d. of monolayers of a C8F17-thiol on gold was 1.48 * 1015 F atoms/cm2, whereas for PTFE a value of 1.24 * 1015 F atoms/cm2 was obsd. This difference was explained by the different tilt angles of the thiol on gold and PTFE chains with respect to the surface normal. Both a configurational and a mol. interpretation on the perfluorinated thiol monolayer on gold are given.