Apparent rate constants for the tert-butoxide promoted Claisen-like condensation of a series of N1-acetyl-N6-aroyl-2,5-dithio-3,4,7,8-tetramethylglycolurils (9a9f) to give N1-(3′-aroyl-3′-oxopropionyl)-2,5-dithio-3,4,7,8-tetramethylglycolurils (10a10f) were determined by UV spectroscopy. Overall rate accelerations of 3.5- to 18-fold were found relative to the corresponding reactions of the 2,5-dioxo compounds (7a7f). Analysis of the Hammett plot for 9 and comparison with that for 7 shows that the key CC bond-forming step, where the enolate of the acetyl group of the substrate attacks the aroyl carbonyl group, is accelerated by the thio substitution. For electron-withdrawing substituents in the aroyl group, the acceleration is sufficient to make this step nonrate limiting: the Hammett ρ value drops from approx. 1.5 for electron-donating groups to 0.27 for electron-withdrawing groups. Deuterium substitution in the acetyl group reduces the rate slightly, a result consistent with a slow but partially reversible first step in which substrate is deprotonated. A similar acceleration and isotope effect are found when diacetyl glycoluril (2) and diacetyl dithio glycoluril (5) are compared. The implications of these results are discussed.Key words: glycoluril, Claisen condensation, kinetics, mechanism.