The kinetics of the thermal substitution of 1,5-cyclooctadiene (COD) from [Mo(CO)4(eta2:2-COD)] (1) by bis-(diphenylphosphino)methane (DPPM) and the formation of [Mo(CO)4(DPPM)] (6) was studied by quantitative FT-IR spectroscopy. The reaction rate exhibits first-order dependence on the concentration of 1, and the observed rate constant, k(obs), depends on the concentrations of both leaving COD and entering DPPM ligand. From the evaluation of data collected, one can propose a mechanism in which the rate-determining step is the cleavage of one of two Mo-olefin bonds. A rate law is derived from the proposed mechanism. The evaluation of the kinetic data gives the enthalpy of activation DELTAH* = 80 (+/-1) kJ.mol-1 and the entropy of activation DELTAS* = -55 (+/-2) J.mol-1.K-1. The negative entropy value obtained tends to suggest an associative mechanism in transition states. Material balance and yield of product do not show any significant depletion implying that there is no decomposition in noticeable amounts during the substitution reaction.