Instantaneous concentrations of reactive species are simultaneously measured in a planar liquid jet with a second-order chemical reaction A+B -> R to investigate the statistical properties of the chemical reaction rate and the validity of models which have been proposed for concentration correlation. The jet flow contains the reactant A, and the ambient flow contains the reactant B. The results show that the concentration correlation of the reactants makes a negative contribution to the mean reaction rate, and this contribution is important in the downstream direction. The concentration correlation changes owing to the chemical reaction. The effects of the chemical reaction on the concentration correlation change with the flow location and the Damkohler number. The concentration correlation predicted by the Toor's model and the three-environment model are compared with the experimental results. The results show that these models fail to accurately estimate the concentration correlation. (C) 2014 American Institute of Chemical Engineers