Many chemical reactions can only take place after a diffusion process involving one of the reagents. This work examines how a reagentAdiffuses into a cylindrical fiber containing a uniform distribution of impuritiesB, when an irreversible second‐order reactionA+B→ABcan take place. The reaction is considered to be the rate‐controlling process. The determination of the different concentration profiles requires the solution of a system of nonlinear partial differential equations for which an approximate solution is derived. It is shown how the diffusion profile of the reagent is modified by the reaction and how the formation of the reaction product depends on the diffusion process. This interdependence is illustrated by some specific examples. For small values of time the concentrations of the diffusing substanceAlie below the values one would expect if no reaction took place. This difference depends on the reaction constantkand the number of impurities. The analysis of the differences between in‐ and out‐diffusion experiments is one way of measuring the influence of the reaction on the diffusion process. One possible application of the present model is the description of hydrogen diffusion and hydroxyl formation in optical fibers.