The apparent density of the adsorbate and the adsorption isotherm have been measured in the systems carbon tetrachloride – charcoal and diethyl ether – charcoal. In both, the apparent density of the adsorbate is lower than the normal liquid density. These abnormally low values cannot be explained on the basis of the coexistence of 'vapor' and 'liquid' adsorbed phases. The results are consistent with the concept of the adsorbatc's blocking-off a portion of the internal voids of the charcoal. The Langmuir equation represents the isotherm only when the quantity adsorbed exceeds two-thirds of the saturation value. The Harkins–Jura equation is valid at low relative pressures in the carbon tetrachloride – charcoal system, but not in the diethyl ether – charcoal system. Neon is adsorbed appreciably on charcoal at 25 °C. and therefore cannot be employed to examine the effect that the gas being used for the measurements has on the apparent density of the adsorbate. Until more accurate data are available for the adsorption of helium on charcoal, both in the absence and presence of other adsorbates, the density of charcoal and the density of the adsorbate measured by helium displacement remain suspect. In view of the uncertainties, the utility of apparent density values of adsorbates in checking the concepts of the various theories of physical adsorption is limited.