a-chymotrypsinogen is a precursor, or zymogen, of chymotrypsin. Chymotrypsin fulfils the role of breaking down proteins into smaller peptides in the digestive tract. It also displays an interesting property in that, when it unfolds due to thermal denaturation, it is able to refold when cooled back to room temperature. UV/Vis spectroscopy with a Peltier accessory can be used to monitor changes in biological and biopharmaceutical samples with temperature. This application brief describes the procedure for measurement of 0.5 mg/mL solution of a-chymotrypsinogen in 10 mM NaCl buffer using the PerkinElmer LAMBDA® 365+ UV/Vis spectrometer with Peltier accessory.