AbstractObserved second‐order rate constants for the hydroxydechlorination of 1‐chloro‐2,4‐dinitrobenzene (2), 2‐chloro‐3,5‐dinitrobenzoate ions (3) and 4‐chloro‐3,5‐dinitrobenzoate ions (4) in micelles of cetyltrialkylammonium bromide (C16H33NR3Br, where R = Me, Et,n‐Pr andn‐Bu) are reported. For substrate 2, the observed catalysis increased as the size of the micellar head group was increased. This was shown to be primarily due to an increase in the rate of reaction in the micellar pseudo‐phase (k 2M). For substrates 3 and 4, the observed catalysis decreased as the size of the micellar head group was increased. Much smaller changes ink 2Mwere observed in these reactions, which lead to a dianionic intermediate which is more sensitive to polarity effects at the micelle surface than is the monoanionic intermediate formed from 2. These results support the contention that as the size of the micellar head group is increased, water is squeezed away from the micelle surface, resulting in a le