<div class="htmlview paragraph">US Navy submarines have used Hopcalite catalyst (MnO₂, CuO) for over 30 years to oxidize carbon monoxide, hydrogen, and many trace organic species. During that time, the material has been manufactured by the same company, and the performance of fresh Hopcalite has never shown significant variation. Recently, a new lot of Hopcalite was tested and found to be 300% more active toward one specific refrigerant than measured in previous lots. In life support applications, unexpectedly high activity can generate hazardous products that endanger crew and equipment. This paper will describe and interpret the tests performed by the Naval Research Laboratory and Naval Surface Warfare Center to investigate different Hopcalite samples.</div>