The decomposition behavior of aqueous Ni(CO)(4), NaCo(CO)(4), and (eta(5) -C5H5)Co(CO)(2) was studied by infrared spectroscopy using a titanium flow reactor with sapphire windows. A pressure of 275 bar was applied to prevent gas-liquid phase separation. Ni(CO)(4) was investigated in the 50-270 degreesC temperature range with selected partial pressures of CO in the range of 0-60 bar. Without added CO, the Arrhenius parameters were E-a = 10.4 +/- 1.3 kcal/mol and ln(A, s(-1)) = 10.4 +/- 1.9 in the 50-90 degreesC range where a first-order rate law was followed. Complete decomposition took place at 170 degreesC within 23 s. The addition of excess CO raised the decomposition temperature. Aqueous Ni(CO)4 could be detected for at least 5 s at 270 degreesC with a 60 bar partial pressure of CO. The isoelectronic ion Co(CO)(4)(-) was found to be more stable than Ni(CO)(4). Its decomposition was characterized in the 200-240 degreesC range without added CO. A first-order rate law was followed up to 50% decomposition, yielding E-a = 29.9 +/- 3.7 kcal/mol and In(A, s(-1)) = 25.9 +/- 3.8. The greater robustness of Co(CO)(4) compared to Ni(CO)(4) is attributable to the strength of the metal-carbon bond. (eta(5)-C5H5)Co(CO)(2) decomposed in the 220-240 degreesC range to Co(CO)(4)(-) and a considerable amount of insoluble residue. Decomposition reactions for the three studied metal carbonyls were formulated.