The adsorptive interaction of 1-butyne and 1-butene with a highly dehydrated pyrogenic silica system has been studied to understand the thermodynamic behavior of the adsorption process by the application of the Langmuir model and of the Van't Hoff equation. In situ FTIR spectroscopy allowed the characterization of the adsorption phenomenon in terms of involved surface sites, gas-volumetric determinations yielded quantitative information relative to the adsorption process, and microcalorimetric results allowed the comparison between calculated and experimental data. K, and Delta(ads)G degrees were obtained from Langmuir's model application; Delta(ads)H data were obtained from the Van't Hoff equation and by the isosteric heats method and were compared with experimental values. The virtual constancy of Delta(ads)H with equilibrium pressure and surface coverage (Langmuir model) allowed us to obtain the Delta(ads)H values and, consequently, the Delta adsS degrees values for the systems of interest.