We report the study of a liquidlike phase that is found in dilute NaCl aqueous solutions frozen at temperatures below the liquid-to-solid-phase transition temperatures of H2O and NaCl.2H(2)O. There is strong evidence that heterogeneous reactions of gases with halides in liquid layers on ice are the source of halogen radicals that destroy the lower tropospheric ozone, and a subeutectic brine phase is thus of particular relevance to discussions of atmospheric composition and its dependence on the chemistry of polar marine ice and snow. The fractions and concentrations of water and NaCl in this subeutectic quasi-liquid phase were measured by H-1 and Na-23 NMR spectroscopy, and the experimental results compared to predictions derived from an equilibrium thermodynamic analysis. The temperature dependence of the salt concentration is well-described by the equilibrium theory for temperature ranges where ideal solution behavior holds; for lower temperatures, where the observed salt concentration increases and deviations from ideality emerge, the predicted concentrations are generally higher than experimental measurements.