Electron transfer in collisions between dipole-bound CH3CN- and C2H3CN-negative ions and the attaching targets SF6 and CCl4 is examined using a Penning ion trap. The data suggest that although it is reasonable to picture the initial electron capture in terms of a free-electron model, the reaction rate constants are somewhat smaller than indicated by this simple model. This results because the negative ions are rather compact and the reaction rate is limited by their geometrical size, i.e., by the 'hard-sphere' collision cross-section. (c) 2005 Elsevier B.V. All rights reserved.