A bimolecular chemical reaction (reactant1+reactant2→products) in laminar Poiseuille flow is experimentally observed using a spectrophotometer. The reaction rate (rm) follows the second-order rate law; that is, rm=&kgr;c1c2, where cm (m=1,2) are the reactant concentrations. The reaction rate constant &kgr; is independently estimated by monitoring the reaction kinetics in a completely mixed batch reactor using the stopped-flow technique. In the reactive transport experiments, the reactants are introduced in a tube and are initially separated by a sharp interface. The variation of the fluid velocity over the cross section of the tube causes the concentrations of the reactants to vary around their cross-sectional average values (c¿m). These spatial variations in the concentrations (cm') influence the overall reaction rate. The cross-sectional average reaction rate is given by r¿m=&kgr;c1c2=&kgr;(c¿1c¿2+c1'c2')=&kgr;(1+s)c¿1c¿2, where s=c1'c2'/(c¿1c¿2) is the segregation intensity. The experimentally observed breakthrough concentration of the product is in agreement with a numerical model that accounts for the effects of the segregation intensity. On ignoring the influence of the segregation intensity, the predicted product concentration substantially exceeds the experimental observations. This shows that for initially non-overlapping reactants the segregation intensity is negative (s<0) and that the overall chemical transformation rate in flowing systems can be significantly different from that implied by substituting the mean concentrations in the expression for the reaction rate. ¿ 1998 American Geophysical Union