Water-soluble fac-[Re(CO)(3)(L,L'-Bid)(X)] (L,L'-Bid = tropolonato, X = H2O, methanol) complexes have been synthesized, and the aqua and methanol substitution reactions were investigated in water (pH range 6.3-10.0) and methanol, respectively, and compared. Thiocyanate ions were used as monodentate entering ligand. The complexes were characterized by UV-vis, IR. and NMR spectroscopy. The crystal structures of the complexes [NEt4] fac-[Re(Trop)(CO)(3)(H2O)]center dot NO3 center dot H2O (reactant) andfac-[Re(CO)(3)(Trop)(Py)], a substitution product, are reported. Overall it was found that the aqua substitution of fac-[Re(CO)(3)(Trop)(H2O)] is about 10 times faster than the methanol substitution reaction for fac-[Re(CO)(3)(Trop)(MeOH)], with forward and reverse rate and stability constants [k(1) (M-1 s(-1)), k(-1) (s(-1)), K-1, (M-1)] for thiocyanate as monodentate entering ligand as follows: fac-[Re(CO)(3)(Trop)(H2O)] = 2.54 +/- 0.03, 0.0077 +/- 0.0005, 330 +/- 22/207 +/- 14 and fac[Re(CO)(3)(Trop)(MeOH)] = 0.268 +/- 0.002, 0.0044 +/- 0.0002, (61 +/- 3)/(52 +/- 4). The activation parameters [Delta H-k1(double dagger)(kJ mol(-1))] Delta S-k1(double dagger) (J K(-1)f mol(-1))or the aqua and methanol complex respectively are 56.1 +/- 0.7, -49 +/- 2 and 64 +/- 1, -43 +/- 5.