Tarnishing of silver by NOx was investigated by interrogation of surface plasmon resonance (SPR). The experimental setup employed the Kretschmann configuration with a 50-nm Ag film deposited on a glass slide with a gas flow-through cell sealed to it. Experiments were carried out for four different NOx concentrations (9, 53, 210, 1015 vpm). Time response curves of reflectance recorded at a fixed angle of incidence exhibited angular displacement of the SPR minimum as a result of growth of a dielectric layer on top of silver. A set of such curves were recorded for each concentration of NOx by continuously sweeping 10 angles of incidence. Kinetic constants were extracted from nonlinear least-square fits using Fresnel’s equations combined with a first-order kinetic law. The process of silver tarnishing by NOx was found to obey the assumed kinetic law. Results demonstrate that time-dependent SPR is a potential technique for investigating the kinetics of surface reactions.