PHTP (the fully trans isomer of perhydrotriphenylene is a host substance for many different inclusion compounds. The X-ray crystal structure of the PHTP-n-heptane adduct is described in detail; it must be considered as a representative term of a large isomorphous series, where both linear polymers (polyethylene, polyoxyethylene, cis- and trans-1,4 polybutadiene...) and small linear molecules (n-hydrocarbons, n-ethers, n-carboxylic acids...), and also some more bulky molecules (iso-octane, CCl4) may be included. The host structure consists of stacks of superimposed molecules, leaving a hexagonal pattern of nearly cylindrical channels, to be filled by the guest substance (a = b = 14.40; c = 4.78 Å; sp. group P63/m). The above picture is substantially unvaried for all the PHTP inclusion compounds, even if more complex structures have been observed (see Notes II and III of this series of papers). The same is true for a kind of statistical disorder, consisting in the mutual substitution of d and l molecules in the same crystal sites. From the characteristic streaks appearing in the X-ray spectra the channel periodicity among n-hydrocarbon molecules has been determined from C5 to C32; there results that the small molecules are appreciably twisted, tending to the characteristic planar shape with increasing molecular weight.