The [2+2] cycloreversion reaction of formylcyclobutane radical anion (c-C4H7-CHO.-) has been investigated at the UB3LYP level with the augmented Dunning's correlation-consistent polarized valence double-zeta basis set supplied with four even-tempered sp shells. Very diffuse p-pi*-like singly occupied orbitals (SOMO) are found for the c-C4H7-CHO.- and product CH2CHCHO.- radical anions, necessitating the use of a rather diffuse basis set for mechanistic study. The respective electron affinities of c-C4H7-CHO and CH2CHCHO are calculated to be 5.4 and 16.1 kcal/mol, showing the ability to bind an extra electron. The intermediate structure (.)(CH2)(3)CHCHO- is found to be a valence-bound distonic anion apt to the elimination of C2H4. The present two-step "rotating" cycloreversion mechanism for c-C4H7-CHO.- is formally similar to the biradical one for neutral cyclobutane structures, but with evidently lower potential barrier. For efficient electron-attachment catalysis, the extra electron should be trapped by suitable functional groups in some orbitals with substantial overlap with the sigma*-orbitals of the cyclobutane structure.