In a reactor and process for the photopromoted catalytic decomposition of compounds in a fluid stream, the area of a catalyst support structure and/or the amount of catalyst on the support structure is varied as a function of distance from a light source to enhance quantum efficiency. In a preferred embodiment, the support structure has a surface area per unit of reaction chamber volume which increases with distance from the light source, or the amount of catalyst on the support structure per unit area increases with distance from the light source to maintain the light intensity on the catalyst within a desired range regardless of dispersion and/or absorption effects. In another embodiment, the support structure includes a plurality of curved plates which may, for example, be spherical, cylindrical or helical.