It is shown that when two fluids are mixed, the correlation between the widths of adjacent lamellae has a significant effect on the yield of a two-step reaction. In particular, positive correlation between the widths of neighbouring lamellae leads to high yields. Lamellar structures are then generated for two idealized fluid mechanical systems and this correlation is quantified. A simulation of the reaction and diffusion in a lamellar system produced by a model flow is performed. The implications for predicting the yield in realistic mixing processes are discussed.