Ab initio calculations [at the MP2 Fc/6-31+G(d) level of theory] indicate that the barriers to the transition states for the competitive five and six centre S_Ni cyclisation processes of the 4,5-epoxypentoxide anion are both 48 kJ mol⁻¹. Experimental studies show that (1) in solution, the 4,5-epoxypentoxide anion cyclises (and at the same time opens the ethylene oxide ring) to give tetrahydrofurfuryl alcohol [tetrahydro-2-furanmethanol (the product of the five-centre cyclisation)] as the predominant product on workup, and (2) collision activation of the 4,5-epoxypentoxide anion in the gas phase gives the tetrahydro-2-furanmethoxide anion as the exclusive product. Since the computed barriers for the cyclisations proceeding through five- and six-membered transition states are comparable, frequency factors (Arrhenius A factors) must be controlling the courses of the kinetically controlled reactions. A comparison of the calculated harmonic vibrational partition functions for the two transition states confirms a higher value of A (and hence a higher rate) for the reaction proceeding via the five-membered transition state.