Key kinetics parameters governing the partitioning of chlorine species in the Antarctic polar stratosphere were retrieved from 28 days of chlorine monoxide (ClO) microwave radiometer measurements made during the late-winter/early spring of 2005 at Scott Base (77.85° S, 166.75° E). During day-time the ratio of the photolysis rate, <i>J</i>, and the dimer formation rate, <i>k</i><sub>f</sub>, determines the partitioning of ClO and its dimer, chlorine peroxide (ClOOCl). This ratio <i>J/k</i><sub>f</sub> remains uncertain and, as a complement to ongoing laboratory studies, the goal of this work is to provide a constraint on that uncertainty based on observations of ClO profiles in the Antarctic. First an optimal estimation technique was used to derive <i>J/k</i><sub>f</sub> ratios for a range of <i>K</i><sub>eq</sub> values. The optimal estimation forward model was a photochemical box model that takes <i>J</i>, <i>k</i><sub>f</sub>, and <i>K</i><sub>eq</sub> as inputs, together with a priori of activated chlorine (ClO<sub>x</sub>=ClO+2×ClOOCl), ozone, temperature and pressure profiles. JPL06 kinetics are used as a priori in the optimal estimation and for all other chemistry in the forward model. Using the more recent JPL09 kinetics results in insignificant differences in the retrieved value of <i>J/k</i><sub>f</sub>. A complementary approach was used to derive the optimal kinetic parameters; the full parameter space of <i>J</i>, <i>k</i><sub>f</sub>, <i>K</i><sub>eq</sub> and ClO<sub>x</sub> was sampled to find the minimum in differences between measured and modelled ClO profiles. Furthermore, values of <i>K</i><sub>eq</sub> up to 2.0 times larger than recommended by JPL06 were explored to test the sensitivity of the <i>J/k</i><sub>f</sub> ratio to changes in <i>K</i><sub>eq</sub>. The results show that the retrieved <i>J/k</i><sub>f</sub> ratios bracket the range of 1.97 to 1.23×(<i>J/k</i><sub>f</sub>)<sub>JPL06</sub> over the range of <i>K</i><sub>eq</sub> values considered. The retrieved <i>J/k</i><sub>f</sub> ratios lie in the lower half of the large uncertainty range of <i>J/k</i><sub>f</sub> recommended by JPL06 and towards the upper portion of the smaller uncertainty range recommended by JPL09.