Using the water-soluble naphthalene carrier of singlet oxygen NDPO2, we have shown that pure singlet oxygen is able to inactivate enveloped viruses (human immunodeficiency virus type 1, herpes simplex virus type 1, cytomegalovirus, vesicular stomatitis virus), but has no effect on non-enveloped viruses (adenovirus and poliovirus 1). These results are related to the experiments on photoinactivation of viruses by hydrophobic photosensitizers (merocyanine 540, hypericin, phthalocyanines, hematoporphyrin and benzoporphyrin derivatives) and they strengthen the hypothesis that singlet oxygen plays a predominant role in this process.