Abstract Measurements of line-broadening in NMR spectra as a result of reactions involving the monomer (COD)Rh(L) in CDCl 3 are reported (COD = cyclooctadiene and L = As[ p-(C 6H 4N(CH 3) 2] 3). In the temperature region −20° to +25° the monomer exchanges L with the dimer [(COD)RhCl] 2 in the NMR time-scale. The kinetic characteristics differ from those previously found with L = triphenylarsine, thus showing that small variations in the ligand may give rise to completely different concentration dependences. The kinetic behaviour in fact resembles that found with the rhodium complex of triphenylphosphine, i.e. a reaction between the monomerand the dissociated dimer is observed. Almost the same parameters are found for the two systems, but the temperature required in the case of triphenylphosphine is slightly higher.