We performed positron annihilation lifetime spectroscopy experiments at Teflon AF (R)/silicon interfaces as function of the positron implantation energy to determine the free volume hole size distribution in the interfacial region and to investigate the width of the interphase. While no interphase was detected in very short chained solvent-free, thermally evaporated Teflon AF (R), an interphase of some tens of nm in extension was observed for high molecular weight spin-coated Teflon AF (R) films. Influences of the native oxide layer on the data evaluation could be ruled out. (C) 2010 Elsevier Ltd. All rights reserved.