Bioconjugates made with the thiol-reactive eosin-5-maleimide can be used as phosphorescent probes or as photosensitizers. With its high quantum yield (∼0.57) for singlet oxygen generation, eosin and its conjugates can be used as effective photooxidizers of diaminobenzidine (DAB) in high-resolution electron microscopy studies and in correlated fluorescence and electron microscopy applications.