The presence of snow can greatly affect the composition of the overlying atmosphere. Furthermore, understanding the chemistry of compounds in a snowpack provides insight into the cycling and fate of pollutants in the environment. In this work, we focus on the photochemical production and reactions of singlet molecular oxygen (O₂(¹Δ_g), hence written ¹O₂*) on ice. While reactions of ¹O₂* are important in surface waters and atmospheric drops, neither the concentrations nor the chemical behavior of ¹O ₂* on ice have yet been determined. In illuminated samples, we found concentrations of ¹O₂* could be greater than 10,000 times higher on ice than in identical, but liquid solution. This enhanced concentration is a result of freeze-concentration, in which solutes/impurities are excluded to liquid-like regions (LLRs) at the surfaces and grain boundaries of, or are trapped in inclusions within, forming ice crystals. For ¹O₂*, which is a reactive intermediate formed after an organic chromophore (i.e., sensitizer) absorbs light and subsequently transfers energy to dissolved oxygen (O₂), freeze-concentration results in elevated concentrations of sensitizers in the liquid-like regions. However, the dominant sink (water) remains essentially unchanged. Thus, ¹O ₂* kinetics on ice are greatly enhanced relative to liquid solutions. If the total solute concentration of a pre-frozen solution is sufficiently high (greater than ∼1 mM) and an ice sample is above the eutectic temperature of the mixture, we find that freezing-point depression can very accurately describe the observed enhancement of ¹O₂* in liquid-like regions. Finally, we found that for water-soluble pollutants, such as furans or aromatic amino acids, ¹O₂* is likely an important oxidant, perhaps rivaling the importance of hydroxyl radical for certain classes of pollutants.