The effect of freezing oil a variety of acidified and neutral, nitrite ion and halide-containing mixtures has been investigated using UV/vis spectroscopy. Several trihalide ions were formed and monitored, including I2Cl-, I2Br-, and IBr2-. A mechanism to explain the observations is given in terms of steps involving INO and the nitroacidium ion, [H2ONO](+). The transformation of sea salt components to specific trihalide ions by freezing represents a potentially important process in a polar atmospheric context. This is because the dichloro- and dibromo-trihalide ions can release chlorine- and bromine-containing gases. which are key intermediates in ozone destruction.