Previous work on CuP 2 single crystals does not comment on their stability under ambient conditions. Simple observations on our CuP 2+x films indicate that the as-deposited amorphous precursors are less stable than polycrystalline CuP 2 films after annealing. This may be due to the extra P present before annealing and to the higher energy associated with the amorphous state (Fig. ). The as-deposited films start to show visual signs of reaction with ambient air a few hours after deposition. Once the films have been annealed, their bulk properties appear to be stable for at least a few days of exposure to ambient conditions. However, their visual appearance and electrical conductivity after several months of storage in a vacuum-packed bag followed by a few days in air are both altered. It is likely that the high sputter pressure necessary to obtain a P/Cu ratio above 2 in our growth setup (see Supporting Information) has a negative influence on stability, because films sputtered at high pressure are generally more porous and more air sensitive due to their higher surface area. It is currently not clear if stability issues are a consequence of the porous morphology or if they are an intrinsic feature of CuP 2 films. The stability of CuP 2 films sputtered at lower pressures or deposited by other techniques should be further investigated.