AbstractThe high melting and most easily crystallizable products of the isomer mixtures obtained by addition chlorination of 1,5‐hexadiene and cis,cis‐2,6‐octadiene were found, by X‐ray analysis, to be the meso stereoisomers (RS)‐1,2,5,6‐tetrachlorohexane (TCH) and (RRSS)‐2,3,6,7‐tetrachlorooctane (threo‐disyndiotactic structure) (TCO) respectively.The carbon atoms in both structures are in a nearly planar zigzag, while the side chlorine atoms are in a gauche position in respect of the carbon atoms. These molecular structures are compared with those of the crystalline chain segments obtained by addition chlorination of cis and trans poly(1‐alkenylenes). The crystals of TCH are monoclinic, space group P21/n, with Z=2 in a unit cell having dimensions: a=9,02±0,03, b=10,09±0,03, c=5,23±0,02 Å β=100°12′±20′.The crystals of TCO are monoclinic, space group P21/n, with Z=2, in a unit cell of dimensions: a=10,97±0,03, b=8,66±0,02, c=6,27±0,02 Å, β=101°55′±20′. The X‐ray analysis has been performed with a Stoë‐automated Weissenberg camera. The structures have been refined by least‐square techniques to R=0,06 for 1076 reflections in the case of TCH and R=0,078 for 973 reflections in the case of TCO.