The kinetics of the ligand substitution reactions between Co(CO) 3 NO and P(C 6 H 5 ) 3 As(C 6 H 5 ) 3 ,Sb(C 6 H 5 3 1,2 bis(diphenylphosphino)ethane has been studied. For the first three ligands it was found active a kinetic two term rate law for the monosubstitution reaction of the type v = (k 1 + k′[L]) [Co(CO) 3 NO], coming from a comaparable contribution of a first- and a second-order path. There is evidence, also from activation enthalphy and entropy data, that the first-order path be a dis sociative one; the k 1 values for all three ligands are almost coincident in a reasonably wide range of temperature. The second-order path is assigned as an associative one, the value of its overall rate constant k′ being affected by ligand variation in the order P(C 6 H 5 ) 2 CH 2 CH 2 P(C 6 H 5 ) 2 > P(C 6 H 5 ) 3 »As(C 6 H 5 ) 3 .Sb(C 6 H 5 ) 3 consequently, the relative contribution of the first-order path to the overall reaction rate increases for the last three ligands in the reversed order, mean figures for a ligand concertration of 10 −1 M being 0.8, 36 and 85% resp. with P(C 6 H 5 ) 3 and Sb(C 6 H 5 ) 3 . Such a behaviour has been interpreted as a function of the basicity. π-acceptor ability and polarizability of the ligands concerned, the first two factors appearing in these cases to prevail. Also the first-order chelation process with 1,2-bis(diphenylphosphino)ethane, following monosustitution, appears from thermal data to be an associative one.