Deactivation of triplet-excited meso-tetra(N-methyl-4-pyridyl)porphine on the surface of dihexadecyl phosphate vesicles was measured by pulsed-laser transient spectrophotometry. Both unimolecular and triplet-triplet annihilation pathways were identified from the concentration dependence of the decay profiles. The data were analyzed using the Monte Carlo method to obtain a surface diffusion coefficient for the adsorbed porphyrin of D approximate to 8 x 10(-9) cm(2)/s at ambient temperatures. The diffusion coefficient underwent a discontinuous change at the major phase-transition temperature of the vesicles, exhibiting an Arrhenius activation energy of E-a = 31 kJ/mol below 62 degrees C and 7.9 kT/mol above 62 degrees C. Comparisons of Monte Carlo simulations to approximate kinetic equations for diffusion-controlled bimolecular reactions confined to spherical surfaces indicate that the latter can be used to estimate diffusion coefficients for reactions on particles that are the dimensions of small unilamellar vesicles.