The resistance of a thin film resistor can be considered as consisting of three parts: 1) the resistance of the resistor material, 2) the resistance of the termination material, and 3) the interfacial resistance. The aging of the interfacial resistance can dominate the aging of low valued resistors, especially under corrosive conditions. The interfacial resistance using a distributed parameter analysis is treated and a figure of merit which can be used to describe the aging of the interface is defined. Also, a sensitive method of measuring this quantity is introduced and a sampling of data on several different termination material systems is presented. The best results were obtained with Ti-Pd-Au. The conclusions drawn from the figure of merit are corroborated by adhesion and thermocompression bond strength studies.