Laboratory studies have demonstrated coupled reduction-oxidation (redox) reactions of included ions in free-falling ice crystals growing in a supercooled liquid water cloud and during the linear freezing of dilute solutions. These reactions are hypothesized to occur as a consequence of the separations of ions of opposite signs at the interfaces between the growing ice phase and the liquid or liquidlike solution layer. The potential determining ions in the ice-water system are hydronium. and hydroxyl ions (i.e., the sign of the charge in the growing ice phase is pH dependent). Changes in the solution pH during the growth of single crystals or freezing of dilute solutions can occur by hydrolysis of reaction products and will change the magnitude of the potential and/or cause a reversal of the charge orientation. The redox reactions occur by electron transfer from oxidizable ions in the solution phase to a positive ice phase, or from a negative ice phase to reduceable ions in the solution phase. They are concentration-dependent reactions, solutions with solute concentrations above about 10(-3) M do not develop potentials. This concentration is also that at which the electrochemical double layer at the ice-solution interface becomes compact. The reduction of carbonate and bicarbonate ions and of absorbed carbon dioxide during the growth of ice crystals containing strong base-weak acid salts was shown to yield formate ion and formaldehyde.