The diffracted-light signals produced in transient grating experiments by phthalocyanine dyes, candidates for photodynamic therapy agents, are shown to be strongly dependent on the presence of oxygen in solution. Data indicate a facile transfer of energy from the excited dye to oxygen in solution to form excited (1)Delta(g) oxygen. The efficiency of excited-oxygen production can be determined by recording the thermal mode of wave motion in a transient grating experiment. The triplet-state lifetimes and quantum efficiencies for the formation of (1)Delta(g) oxygen determined from the experiments are found to depend on structural details of the dyes, which can be rationalized by considering pi-orbital interactions. The salient features of the transient grating method for evaluating type II photodynamic therapy agents are that the method requires no chemical species to be added to the dye solution that complicate the photochemistry and that the method is sensitive enough that experiments can be carried out at micromolar concentrations.