AbstractKinetic studies of the addition of the aminoxyl radical TEMPO (2,2,6,6‐tetramethylpiperidinylaminoxyl, TO·) to the 4‐substituted phenylketenes 8 (4‐RC6H4CHCO, R = NO2, CN, Cl, H, CH3, CH3O) and to 3‐pyridylketene (15) reveal a correlation of logk2(TEMPO) with the Hammett sigma parameters of the substituents (including the aza substituent of 15), with rho = 1.58 (r = 0.94). A better correlation is obtained with the rate constants for hydration of the same substrates: logk2(TEMPO) = 1.64logk(H2O)−5.76 (r = 0.98). These results fit within a previously established correlation of ketene hydration reactivity with aminoxyl radical reactivity, and confirm that the reactivity of TEMPO in additions to ketenes is dominated by the nucleophilic character of the aminoxyl oxygen, with a strong trend of increasing reactivity for more electrophilic ketenes. Copyright © 2006 John Wiley & Sons, Ltd.