Reported is a combined time-resolved optical (TRO) and infrared (TRIR) spectroscopic investigation of the flash photolysis of MO(CO)6 in cyclohexane solution. TRIR studies using 308 nm excitation led to transient bleaching of the strong v(CO) band at 1987 cm(-1) of Mo(CO)(6) and appearance of new bands at 1931 and 1964 cm(-1) attributed to Mo(CO)(5)(SOl). Using a high pressure/variable temperature flow cell, the kinetics of back reaction with CO (k(CO)) to regenerate the hexacarbonyl was studied over the P-CO range 1-20 atm and at five temperatures. These data gave k(CO) = 4.6 +/- 0.2 x 10(6) M-1 s(-1) (298 K) and the activation parameters DeltaH(CO)(double dagger) = 32.6 +/- 13 kJ/mol and AS -7.3 +/- 11 J mol(-1) K-1 from which an interchange mechanism was proposed. The analogous species seen in the TRO experiment displayed a transient absorbance at 420 nm and analogous kinetics properties although at lower P-CO self-trapping with Mo(CO())6 (to give Mo-2(CO)(11)) is a competitive process. The Mo(CO)(5)(SOl) transient could also be trapped by "PrBr (k(RBr) = 5.3 +/- 0.7 x 10(7) M-1 s(-1)). (C) 2003 Elsevier B.V. All rights reserved.