Physical conditions in the 10-20 km region of the Antarctic stratosphere make the [NO2>/ [NO> ratio particularly sensitive to high chlorine levels in the form of ClO. According to simple known photochemical relationships between NO2, NO, ClO, and O3, high ClO levels of 1 ppbv over Antarctica must be accompanied by large values of the [NO2>/ [NO> ratio. At high ClO abundances, the [NO2>/ [NO> ratio is approximately proportional to the ClO concentration. It is proposed that in situ measurements of the [NO2>/ [NO> ratio could be used to test the high chlorine hypothesis.