Photocatalytic vs. photosensitizing materials to be used for gas-phase photooxidation and photooxygenation, respectively, are described. An overview is given on reactive oxygen species specific to photocatalysis and photosensitization as well as on various kinds of gas-phase reactors and photocatalytic materials in order to illustrate the great number of possible configurations and conditions of use. Some efforts towards standardization of gas-phase photocatalytic reactors devoted to indoor-or outdoor-air treatment are mentioned. Examples for application of singlet oxygen in solvent-free photooxygenation reaction of volatile sulfides are given together with the properties of silica-based photosensitizing materials.