The experimentally measured infrared (IR) reflectance of metals has sometimes been known to depart significantly from that predicted by simple theory. This departure is a complex function of the bulk and surface properties, method of fabrication, age, and temperature history of the metal. This paper presents data which show the effects of the thermal history of a metal on its IR reflectance as a function of these parameters. The overall sample set consisted of copper (bulk and thin film), aluminum (thin film and bulk alloy), silver (thin film), and molybdenum (bulk) mirrors, and each metal was further categorized as to age, fabrication, and if a thin film by substrate. Reflectance measurements at wave-lengths between 9.72 and 0.8 μm, inclusive, were then made for each category in air at room temperature prior to and following a thermal excursion to look for resulting effects. In addition, each category contained a control sample in order to filter out changes due entirely to age and storage, and a “standard” was periodically measured to track any instrumental drifts occurring with time. The data presented herein represent only one thermal excursion, and yet the data show that just one cycle can have complex effects upon the IR reflectance of a metal, depending upon the temperature of the excursion and how and when the mirror was made.