Low-temperature flash photolysis of CpMo(CO)(3)-Co(CO)(4) (A) or CpMo(CO)(3)-Co(CO)(3)(P(n-Bu)(3)) (B) in 3MP glass results in loss of CO as the only IR-detectable photoprocess. Flash photolysis of CpMo(CO)(2)(P(n-Bu)(3))-Co(CO)(3)(P(n-Bu)(3)) (C), however, results in loss of P(n-Bu)(3) as the major photoprocess with loss of CO as a minor process. An initially observed solvento species, 1, produced in photolysis of A at 92 K rearranges to form a symmetrically bridging species, 2. Symmetrically bridging species, 3 and 4, and linear semibridging species, 5 and 6, are observed initially in the photolysis of B at 93 K. When the temperature is raised. 3 and 4 rearrange to form 6 while 5 isomerizes to form another linear semibridging species, 7. When B is photolyzed at 143 K, only 6 and 7 are still observable. All CO-loss intermediates recombine with expelled free CO to re-form the starting compound. If B is photolyzed in the presence of P(n-Bu)(3) at 93 K, the four initially formed intermediates, 3-6, are observed to react with P(n-Bu)(3) when the glass is warmed, yielding compound C through intermediates 8-10. Photolysis of C at 93 K yields initially the same four intermediates, 3-6, as photolysis of B and another intermediate, 11, upon loss of P(n-Bu)(3) and CO, respectively. Some of the four intermediates react with expelled free CO, giving rise to compound B; some react with the lost free P(n-Bu)(3), re-forming C, also through the same intermediates, 8-10. All the experimental observations, together with results from thermal reactions with (CO)-C-13, can be explained by the selective loss of CO or P(n-Bu)(3) from the Mo end of the molecule for B and C, respectively. A detailed mechanism of the photochemical behaviors of the substituted compounds and structures of the observed intermediates (including ring-slippage structures) are proposed. Steric and electronic effects on formation and stabilities of linear semibridging species are discussed.