The kinetics and mechanism of the deprotection (detritylation) of 5′-O-(4,4′-dimethoxytrityl)-2′-deoxythymidine nucleoside catalysed by dichloroacetic acid to give a 4,4′-dimethoxytrityl carbocation have been studied in toluene, dichloromethane and acetonitrile. There is little or no effect of solvent polarity on the equilibrium and rate constants. Entropies of activation are highly negative ∼-105 J K -1 mol-1 and similarly show little variation with solvent. Addition of small amounts of water to the reaction medium reduces the detritylation rate, presumably through its effect on the solution acidity. All observations are compatible with detritylation occurring through a concerted general acid-catalysed mechanism rather than a stepwise A1 process.