The photoactivity of RCo(CO)(4) (R = H, CH3) complexes has been investigated and compared by means of state correlation diagrams connecting the low-lying singlet E-1 (d(Co) -> sigma*(Co-R) and d(Co) -> pi*(CO)) and (1)A(1) (d(Co) -> pi*(CO)) electronic states accessible through UV irradiation, and the low-lying triplet states (E-3 and (3)A(1)), to the corresponding states of the primary products R + Co(CO)(4) and COax + RCo(CO)(3). The electronic absorption spectra have been calculated by time-dependent wave packet propagations on two-dimensional potential energy surfaces describing both channels of dissociation, namely the homolysis of the R-Co and the COax-Co bonds. It is shown that the absorption spectrum of HCo(CO)(4) is characterized by two peaks; the most intense peaks for each set are located respectively at 42 659 and 45 001 cm(-1). The CH3Co(CO)(4) absorption spectrum also gives two sets of signals with maximum intensities found at 42 581 and 51 515 cm(-1) These bands for both molecules are assigned to the two metal-to-ligand-charge-transfer (MLCT; d(Co) -> pi*(CO)) states. Three photoactive states have been determined in both molecules, namely the singlet metal-to-sigma-bond-charge-transfer (MSBCT) states (a(1)E and b(1)E), simultaneously dissociative for both the homolysis of CO and the R-Co bond, and the (3)A(1) (sigma(Co-R) -> sigma*(Co-R)), dissociative along the R-Co bond.