The dimeric Co-2(CO)(6)(AsPh3)(2) molecule contains an unsupported Go-Co bond axial arsines, and equatorial carbonyls. At room temperature, the molecule crystallizes in the R (3) over bar space group [a = 15.473(3) Angstrom, c = 14.240(2) Angstrom] packing in chains along c which are hexagonally arranged in the ab plane. When the temperature is lowered, no significant changes are found in the molecular structure until a second-order phase transition [T-c = 206(3) K] occurs by doubling the c axis [at T = 123 K a = 15.233(4) Angstrom, c = 28.440(6) Angstrom, space group R (3) over bar]. The major molecular conformational changes related to the phase transition are the rotations of Co(CO)(3) and AsPh3 fragments about the 3-fold axes and those of the phenyls about their As-C bond. X-ray studies at 16 different temperatures have been performed in order to describe the details of the small continuous structural changes that occur in the low-temperature phase.