A technique is described wherein substituent-dependent excited-state properties of gas-phase porphyrins can be accurately quantified. Dissociation lifetimes of photoexcited porphyrin cations were measured in an electrostatic ion storage ring. From these data, upper limits for the triplet quantum yield of the following protonated porphyrins were obtained: protoporphyrin IX (0.63 +/- 0.04), tetraphenylporphyrin (0.73 +/- 0.03), tetra(p-methylphenyl)porphyrin (0.75 +/- 0.03), and tetra(p-cyanophenyl)porphyrin (0.71 +/- 0.03). The values compare well with those for porphyrins in solution.