The reaction kinetics between carboxylic-terminal polybutadiene (PBd-COOH) and amino-terminal polydimethylsiloxane (PDMS-NH2) has been studied at the PBd/PDMS interface by measuring changes in interfacial tension during the reaction and inferring concentrations of the PDMS-NH3OOC-PBd complex reaction product by an application of the Gibbs adsorption equation. The time-dependent concentration of complex is obtained from the transient interfacial tension and follows a single-exponential growth function at low surface coverage, indicating first-order reaction kinetics.