The CO exchange on cis-[M(CO)(2)X-2](-) with M = Ir (X = Cl, 1a; X = Br, 1b; X = I, 1c) and M = Rh (X = Cl, 2a; X = Br, 2b; X = I, 2c) was studied in dichloromethane. The exchange reaction [cis-[M(CO)(2)X-2](-) + 2*CO reversible arrow cis-[M(*CO)(2)X-2](-) + 2CO (exchange rate constant: k(obs))] was followed as a function of temperature and carbon monoxide concentration (up to 6 MPa) using homemade high gas pressure NMR sapphire tubes. The reaction is first order for both CO and cis-[M(CO)(2)X-2](-) concentrations. The second-order rate constant, k(2)(298) (=k(obs)/[CO]), the enthalpy, Delta H-double dagger, and the entropy of activation, Delta S-double dagger, obtained for the six complexes are respectively as follows: 1a, (1.08 +/- 0.01) x 10(3) L mol(-1) s(-1), 15.37 +/-0.3 kJ mol(-1), -135.3 +/- 1 J mol(-1) K-1; 1b, (12.7 +/- 0.2) x 10(3) L mol(-1) s(-1), 13.26 +/- 0.5 kJ mol(-1), -121.9 +/- 2 J mol(-1) K-1; 1c, (98.9 +/- 1.4) x 10(3) L mol(-1) s(-1) 12.50 +/- 0.6 kJ mol(-1), -107.4 +/- 2 J mol(-1) K-1; 2a, (1.62 +/-0.02) x 10(3) L mol(-1) s(-1), 17.47 +/- 0.4 kJ mol(-1), -124.9 +/- 1 J mol(-1) K-1; 2b, (24.8 +/- 0.2) x 10(3) L mol(-1) s(-1), 11.35 +/- 0.4 kJ mol(-1), -122.7 +/- 1 J mol(-1) K-1; 2c, (850 +/- 120) x 10(3) L mol(-1) s(-1), 9.87 +/- 0.8 kJ mol(-1), -98.3 +/- 4 J mol(-1) K-1. For complexes 1a and 2a, the volumes of activation were measured and are -20.9 +/- 1.2 cm(3) mol(-1) (332.0 K) and -17.2 +/- 1.0 cm(3) mol(-1) (330.8 K), respectively. The second-order kinetics and the large negative values of the entropies and volumes of activation point to a limiting associative, A, exchange mechanism. The reactivity of CO exchange follows the increasing trans effect of the halogens (Cl < Br much less than I), and this is observed on both metal centers. For the same halogen, the rhodium complex is more reactive than the iridium complex. This reactivity difference between rhodium and iridium is less marked for chloride (1.5:1) than for iodide (8.6:1) at 298 K.