The high perchlorate (ClO 4 −) to chloride (Cl −) ratios observed at the Phoenix landing site, northern polar region of Mars, have been puzzling since detection. However, a lack of understanding of perchlorate-chloride-water systems under cryogenic conditions makes it difficult to assess ClO 4 − /Cl − ratios during deliquescence-related processes. Here we quantitatively evaluate ClO 4 − /Cl − fractionation in deliquescence-induced brines of magnesium-and calcium-perchlorate-chloride salt mixtures under subzero conditions, by measuring solubility data and constructing temperature-dependent thermodynamic models. We find that under specific relative humidity (RH) and temperature (T) conditions, deliquescence of perchlorate-chloride mixtures may form brines with fractionated ClO 4 − /Cl − signatures. Appropriate RH-T, water-limited conditions, and aeolian processes are required to produce and preserve the elevated ClO 4 − /Cl − signatures in soils. Under the present climate, the north polar region can support ClO 4 − /Cl − fractionation and potentially enrich perchlorate for longer periods on global Mars. This highlights the uniqueness of Mars' arctic environment and its implications for modern habitability.