Cobalamins are of widespread importance in biology. Both of the cofactors essential for human metabolism, the organocobalamins coenzyme B-12 and methylcobalamin, are highly photolabile, as are other alkylcobalamins. The alkynylcobalamin phenylethynylcobalamin (PhEtyCbl) and the arylcobalamin 4-ethylphenylcobalamin (EtPhCbl) with "atypical" Co-C-bonds to unsaturated carbons, were recently designed as metabolically inert cobalamins, classified as "antivitamins B-12". The further development of an ideal light-activated or "conditional" antivitamin B-12 would require it to be readily converted by light into an active B-12 vitamin form. Very photolabile "antivitamins B-12" would also represent particularly useful scaffolds for therapeutic light-activated reagents. Here, the photoactive arylcobalamin EtPhCbl and the remarkably photostable alkynylcobalamin PhEtyCbl are examined using femtosecond to picosecond UV-visible transient absorption spectroscopy. PhEtyCbl undergoes internal conversion to the ground state with near unit quantum yield on a time scale < 100 ps and an activation energy of 12.6 +/- 1.4 kJ/mol. The arylcobalamin EtPhCbl forms an excited state with a ca. 247 ps lifetime. This excited state branches between internal conversion to the ground state and formation of a long-lived base-off species with a quantum yield of similar to 9%. Anaerobic steady state photolysis of "light-sensitive" EtPhCbl results in the formation of co(II)alamin, but only with quantum yield <1%. Hence, our studies suggest that suitably modified arylcobalamins may be a rational basis for the design of photoresponsive "antivitamins B-12".