Density and sorption measurements were carried out with ethane, carbon dioxide, and their binary mixture with an ethane mole fraction of 0.2003 using a modified gravimetric sorption analyzer. Three solid sinkers with a large surface-to-volume ratio and different (quasi nonporous) surface materials and zeolite 13X (as porous material) were investigated. In the vicinity of the dew point, the density measurements were distorted, thus implying an impact of surface phenomena. Three hypothetical regions were identified for distortion: (1) gas adsorption of a single layer or a few layers at pressures lower than 0.85 × ps, where ps is the dew-point pressure; (2) multilayer gas adsorption followed by the beginning of capillary condensation up to about 0.995 × ps; (3) domination of capillary condensation up to ps. With the goal to deliver a sound data basis for the development of an advanced description of fluid-mixture vapor-liquid-equilibrium through equations of state, we established an empirical method to correct the distorted density measurements. Sorption measurements with the well-investigated zeolite 13X were carried out to confirm the reliable performance of our instrument and to provide a background for the interpretation of the measurements with the solid sinkers. The results of these measurements support the hypothesis of the three regions of surface phenomena and reveal that a gold-plated solid surface reduces the extent of the observed effects.