In an investigation of methylene replacement by small heteroatom insertions the crystal structures of N,N-dioctadecylamine, (C18H37)(2)NH, and dihexadecyl ether, (C16H33)(2)O were determined by electron crystallography and infrared spectroscopy. Crystals of the amine, epitaxially oriented on naphthalene, are characteristic of the analogous odd-chain n-paraffin in its high-energy orthorhombic B-polymorph. Cell constants are a = 7.52(2), b = 5.04(1), and c/2 = 50.0(4) Angstrom and the extinction rules for the 0,k,l reflections (k + l = 2n + 1) are consistent with the space group A2(1)am. Carbon and nitrogen coordinates in a paraffin-like model match observed data reasonably well (R = 0.22). A binary phase diagram reveals that the secondary amine is continuously cosoluble with the n-paraffin of the same chain length (n-C37H76) The ether, crystallized on benzoic acid, is polymorphic. The major monoclinic form, which forms a eutectic solid with n-C33H68. crystallizes in space group Aa with a approximate to 5.59, b approximate to 7.40. and c = 88.66 Angstrom and beta = 116.2 degrees. Its projected crystal structure includes a twisted chain conformation in an oblique layer (R = 0.26). The orthorhombic form in space group A2(1)am (c/2 = 45.1 Angstrom) resembles the odd-chain paraffin and may be continuously co-soluble with it.