A photomicrocalorimeter is described for studies of reactions involving oxygen and moisture-sensitive compounds in organic solvents. This calorimeter was used to investigate the thermochemistry of photochemical substitutions of a carbonyl in [Mn(eta(5)-C5H4CH3)(CO)(3)] by P(OR)(3) (R = Ph or Pr-i). The measured reaction enthalpies in heptane (lambda = 400 nm), Delta H-r = - 15.7 +/- 0.5 and - 18.6 +/- 0.9 kJ mol(-1), respectively, were identified with the bond dissociation enthalpy differences D(Mn-CO)-D[Mn-P(OPh)(3)] and D(Mn-CO)-D[Mn-P((OPr)-Pr-i)(3)] in solution. These results are discussed in terms of the basicity of the phosphites, spectroscopic data of the complexes (CO stretching frequencies), and the available thermochemical data for other transition metal-phosphorus(III) ligand interactions.48241671111118