The rhombohedral unit cell dimensions of the PHTP adducts with cyclohexane (a = b = 25.55; c = 43.02 Å) and with dioxane (a = b = 25.11; c = 28.68 Å) suggest the presence of 9 and 6 PHTP superimposed molecules per c length respectively. The crystal structure of the cyclohexane adduct has been worked out completely; the space group is $R \overline{3}$. Its most unusual feature is the presence of stacks of PHTP molecules whose centres are helicoidally displaced from the trigonal axes of the structure (helix radius = 0.40 Å; 9 residues in 2 pitches). The cyclohexane molecules are arranged along the resulting channels at such levels as to be properly interlocked with the surrounding helices: 7 included molecules per c length are contained in each channel, with a more or less random orientation. The dioxane adduct contains PHTP helices of 6 residues in one pitch, and 5 dioxane molecules per c length in each channel; due to the analogy of its X-ray spectra with those corresponding to the previous adduct, the probable space group is still $R \overline{3}$.