The Solar Heat Gain Coefficient (SHGC) of the opaque portion of a curtain wall system has been determined experimentally. The curtain wall system under investigation consists of wall panels with structural exterior glazing. A 5- inch thick drywall was built in place. This steel stud wall, insulated with fiberglass and sheeted with gypsum board, is backed up against the shadow box of the curtain wall panel, see Fig. 1. The examined building is located on a university campus in the arid climate of the Northern edge of the Sonoran desert. A monitoring system was set up to measure temperatures, solar radiation, and heat flux data. The data show that the temperature in the sealed air cavity of the curtain wall section (the shadow box) peaked above 200°F. The air temperature in the shadow box frequently reached 95°F above the interior wall surface when the wall was exposed to solar radiation. The SHGC calculated from the acquired data was significantly higher than the SHGC used in a computer simulation performed during the design phase. It was also found that the SHGC fluctuates with the temperature elevation in the shadow box. This phenomenon is characterized as a dynamic solar heat gain coefficient.