A joint threshold photoelectron photoion coincidence spectrometry (TPEPICO) and collision-induced dissociation (CID) study on the thermochemistry of Co(CO)(2)NOPR3, R = CH3 (Me) and C2H5 (Et), complexes is presented. Adiabatic ionization energies of 7.36 +/- 10.04 and 7.24 +/- 0.04 eV, respectively, were extracted from scans of the total ion and threshold electron signals. In the TPEPICO study, the following 0 K onsets were determined for the various fragment ions: CoCONOPMe3+, 8.30 +/- 0.05 eV; CoNOPMe3+, 9.11 +/- 10.05 eV; CoPMe3+ 10.80 +/- 0.05 eV; CoCONOPEt3+, 8.14 +/- 0.05 eV; CoNOPEt3+, 8.92 +/-0.05 eV; and CoPEt3+, 10.66 +/- 0.05 eV. These onsets were combined with the Co+-PR3 (R = CH3 and C2H5) bond dissociation energies of 2.88 +/- 0.11 and 3.51 +/- 0.17 eV, obtained from the TCID experiments, to derive the heats of formation of the neutral and ionic species. Thus, the CO(CO)2NOPR3 (R = CH3 and C2H5) 0 K heats of formation were found to be -350 +/- 13 and -376 +/- 18 kJ-mol(-1), respectively. These heats of formation were combined with the published heat of formation Of CO(CO)(3)NO to determine the substitution enthalpies of the carbonyl to phosphine substitution reactions. Room-temperature values of the heats of formation are also given using the calculated harmonic vibrational frequencies. Analysis of the TCID experimental results provides indirectly the adiabatic ionization energies of the free phosphine ligands, P(CH3)(3) and P(C2H5)(3), of 7.83 +/- 0.03 and 7.50 +/- 0.03 eV, respectively.