The thermal substitution kinetics of 1,5-cyclooctadiene (COD) from M(CO)(4)(eta(2:2)-COD) (M = Cr, Mo, W) by bis(diphenylphosphino)alkane, (C6H5)(2)P(CH2)(n)-P(C6H5)(2) (n = 1, 2, 3), was studied by quantitative FT-IR spectroscopy. The reaction rate exhibits first-order dependence on the concentration of starting complex and the observed rate constant depends on the concentrations of both the leaving COD and entering diphosphine ligand. In the proposed mechanism the rate determining step is the cleavage of one of two metal-olefin bonds. A rate-law is derived from the proposed mechanism. The evaluation of the kinetic data gives the activation parameters which tend to suggest an associative mechanism for the molybdenum and tungsten complexes, and a dissociative mechanism for the chromium complexes in transition states. The observed rate constant is found to be dependent on the concentration and the nature of the entering diphosphine ligand.