A similarity law is proposed for the yield from laminar photolytic reactors. In particular, the yield from a fast photolytic reaction is considered in the following three distinctly different reactor geometries: a square channel, an annular gap between concentric cylinders, and Taylor - Couette flow. The similarity law, based on the assumption of a self similar product concentration profile, demonstrates that the reaction rate can best be described with either zero (small A) or first-order (large A) kinetics. Here, the distinction between both cases is determined by the radiation penetration depth, A. Other geometric parameters affecting scale-up are also discussed.