The CO-releasing ability of a diverse library of primary metal carbonyl complexes has been assessed using a deoxymyoglobin-carbonmonoxymyglobin assay. A wide spectrum of rates for the CO-release process was observed in aqueous systems. For octahedral d⁶ complexes, the rate was found to decrease in the sequence FeI₂(CO)₄ > [NEt₄][V(CO)₆] > MnBr(CO)₅ > Cr(CO)₆ implying that CO-release is not controlled by the metal–carbon bond strengths. Within the series, [NEt₄][MX(CO)₅] (M = Cr, Mo, W; X =Cl, Br, I), the rate of CO-release was found to decrease down the group (Cr > Mo > W), whilst within the chromium series a similar trend was observed for the halide (Cl > Br > I). The d⁴ complexes [NEt₄][MI₃(CO)₄] (M = Mo, W) exhibit faster release than their d⁶ congeners. A mechanistic investigation into the [NEt₄][MX(CO)₅] series revealed the intermediacy of [{M(CO)₅}₂(μ-X)]⁻ in the CO-release process and that the hydrolysis of the M–X bond, rather than the intrinsic strength of M–CO bonds, controls the rate of CO-release in aqueous systems.