Earth-abundant zinc phosphide (Zn3P2) holds great promise as a photovoltaic absorber for thin-film applications, owing to its direct bandgap of ∼1.5 eV and its high absorption coefficient in the visible spectrum. Nonetheless, questions remain about the material’s stability in atmospheric environments, given its tendency to react with surrounding water vapor and oxygen. This work presents a comprehensive long-term study to understand how environmental exposure impacts high-quality, monocrystalline epitaxial zinc phosphide. Through a combination of various experimental techniques, such as ellipsometry, Raman spectroscopy, x-ray diffraction measurements, scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, secondary ion mass spectroscopy, and x-ray photoelectron spectroscopy, we reveal that exposure to the atmosphere causes substantial oxidation of the thin film surface, penetrating several tens of nanometers into the bulk material. Finally, we show that degradation can be effectively prevented by applying a thin dielectric layer, such as Si3N4, or by simply storing the unprotected thin films under vacuum. These findings provide valuable guidelines for the proper handling of the material prior to device fabrication.