Photolysis was studied in Taylor - Couette flow, and a similarity law proposed for the formation of a product from a fast photochemical reaction in the presence of laminar Taylor vortices. The concentration of the product formed depended on both the dosage of photons and a reaction-layer thickness, defined by the radiation penetration depth. In particular, for a given flow rate and radiation intensity, the UV photolysis of iodide produced a maximum concentration of the product triiodide, when the radiation penetration depth was equal to the velocity boundary-layer thickness. The latter conclusion provides optimum operating conditions to maximize yield in terms of the Taylor number or frequency of rotation.