Abstract Competitive binding of analytes in solution, to an immobilized receptor, is a preferred method for quantifying molecules in complex mixtures. We have developed a theoretical and experimental framework for the microfluidic competition assay. A mathematical model describes the transient, convection–dispersion of solutes, undergoing equilibrium binding to immobilized receptors, while entrained in a low Reynolds number incompressible fluid flowing through a microchannel. The proposed method involves monitoring of the elution profile of a reference molecule and ligand in the presence of a competitor. The time difference between the two breakthrough curves provides a measure of the unknown concentration of the competitor. Theoretical results illustrate the general method for determining the equilibrium dissociation constant ( K d) of the ligand and competitor, as well as the competitor concentration. Experimental data is presented for the binding of fluorescein labeled insulin and unlabeled insulin to a monoclonal antibody. It is found that the unlabeled insulin binds with higher affinity ( K d = 0.17 μM) than the labeled insulin ( K d = 0.76 μM). The potential advantages of the method and further improvements in the model are discussed.