Industrial wetting situations often involve the presence of a third component near an interface, potentially modifying the local effective capillary balance. We here consider a simplified situation in which a drop is placed on a substrate supporting a thin layer of "contamination." As spreading of the drop to equilibrium occurs, the contaminant diffuses into the drop, modifying the effective substrate/drop interfacial free energy (increasing it, it is assumed). Thus the kinetics of spreading are altered. The essential effect when the final equilibrium contact angle is zero is that of accelerating the process. However, when the final value of contact angle is finite, "overshoot" may occur. The drop spreads beyond its equilibrium state and then retracts to attain its equilibrium state asymptotically at long times,