This paper describes a method for determining C-O stretching frequencies of dicobalt octacarbonyl belonging to D-3d point group. The method is based on the variation of fundamental C-O stretching frequencies and C-O stretching force constants of axially substituted X3MCO(CO)(4) (M=Si, Ge, Sn and Ph; X=Cl; Ph and Et) molecules with the dipole moment of X3M-Co(CO)(4) bond. Frequencies of a(lg)((1)) a(2u)((2)) and e(u) modes have been evaluated from the graph of frequency versus dipole moment, and the frequencies of a(lg)((2)) and e(g) modes determined from the equations derived by using the CO-factored force field. The calculated frequencies have been found to be consistent with the experimental values. With the use of the calculated frequencies, the C-O factored force constants of Co-2(CO)(8) with D-3d symmetry have been calculated. In addition. frequencies of mono-(CO)-C-13 substituted species of Co-2(CO)(8) have been estimated and compared with observed frequencies of the species.