Cyclopentoxide c-C5H9O- undergoes photodetachment to stable cyclopentoxy or the ring-opened 5-oxo-pentan-1-yl radical and dissociative photodetachment, yielding C3H5O and C2H4 photofragments, at both 532 and 355 nm. The adiabatic electron affinity of c-C5H9O- is estimated from the experimental results and ab intio calculations to be 1.5 +/- 0.1 eV. The results show that c-C5H9O- is stable relative to dissociation into C3H5O- and C2H4 by 1.23 +/- 0.07 eV, whereas c-C5H9O is unstable relative to C3H5O and C2H4 by -0.12 +/- 0.12 eV. These results are discussed in terms of the factors affecting the stability of cyclic alkoxides and the corresponding alkoxy radicals. (C) 2002 Elsevier Science B.V. All rights reserved.