This paper describes a planar photoelectrochemical structure consisting of a thin porous layer ofnonmetallized semiconducting powder above a metallic film. Suited to the photocatalysis of redox reactions in an aqueous gas‐phase environment, the properties of the structure are illustrated using the TiO2sensitized photodecomposition of formic acid which was observed to occur at quantum efficiencies as high as 30%. A photoelectrochemical mechanism is proposed which is the powder‐layer analogue of that describing the catalytic activity of dispersed metallized powders. The approach holds potential for fabricating more complex photochemical structures as well as providing new ways of studying powder‐sensitized reactions.