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The effect of solvent composition in aqueous ethanol, trifluoroethanol and hexafluoropropan‐2‐ol on the rate constant and activation parameters for the uncatalysed hydrolysis of 2‐(p‐nitrophenoxy)tetrahydropyran (1) was investigated, and the m(YOTs) value is 0.60. This appreciable but less than maximal value is in accordance with an SN1 mechanism with rate‐limiting ionization. The α‐deuterium secondary kinetic isotope effect (α‐kie) for the uncatalysed hydrolysis of 1 is 1.17 in water (46°C), 1.15 in aqueous trifluoroethanol (50% mole fraction, 70.6°C) and 1.13 in aqueous ethanol (50% mole fraction, 70.6°C). These values correspond to about 1.19 at 25°C, which is characteristic of rate‐limiting ionization in an SN1 reaction and appreciably higher than values for enzyme‐catalysed glycolysis. The α‐kie is smaller under aqueous acidic conditions (1.07, 0.1 mol dm−3 hydrochloric acid, 20.2°C) when 1 hydrolyses with acid catalysis. The previously reported α‐kie for the hydrolysis of 1 in buffered aqueous dioxan (1.063, 25°C) is now seen to correspond to acid‐catalysed hydrolysis. These new results for 1 indicate that transition structures in enzyme‐catalysed glycolyses with α‐kie values of less than about 1.15 at 25°C involve a lower degree of carbenium ion character than has hitherto been assumed. Copyright © 2004 John Wiley & Sons, Ltd.