The crystal structures of the low-melting 1,2- and 1,3-dichlorobenzene isomers in monoclinic space group P21/n and monoclinic space group P21/c, resp., were detd. by x-ray anal. and in situ crystn. techniques. Attempts to predict these structures in advance by force-field calcns. were not successful, although the known crystal structures of two of the three polymorphs of the 1,4-isomer were successfully a posteriori predicted. Calcd. lattice energies were supplemented with estd. lattice-vibrational entropies obtained in the rigid-body approxn. Energy calcns. for actual and virtual crystal structures indicate that the higher m.p. of the 1,4-isomer can be largely attributed to more efficient crystal packing.