Gravimetric isotherms at 25°C for hexane and octane on 15.4 m2/gm PTFE are presented. The major contribution to the measured adsorption at high relative pressure (P/Po > 0.7) is condesnation at points of particle contact. At low and intermediate relative pressures, both multilayer adsorption on high-energy sites and submonolayer adsorption on low-energy sites contribute to the measured adsorption. Adsorption on the high-energy sites, constituting 3% of the surface, is resolved from the isotherm form at low relative pressures (P/Po < 0.015) and application of the Frenkl-Halsey-Hill equation. Adsorption on the low-energy surface is obtained by difference. Lateral interaction energy constants and adsorbate cross-sectional areas are obtained from computer curve fit techniques applied to the Hill-de Boer equation. Adsorption quantities and spreading pressures at saturation vapor pressure are obtained by analytical extrapolation. © 1968.