Chemical composition, core-level line shape, and electron affinity measurements are performed on the Se-terminated (2\ifmmode\times\else\texttimes\fi{}1) and Zn-terminated c(2\ifmmode\times\else\texttimes\fi{}2) ZnSe(100) surfaces. The (2\ifmmode\times\else\texttimes\fi{}1) reconstruction corresponds to a complete monolayer of Se dimers with one filled dangling bond per surface atom. The c(2\ifmmode\times\else\texttimes\fi{}2) reconstruction corresponds to a half-monolayer vacancy structure which leaves twofold-coordinated Zn atoms in the top layer. The 150-meV decrease in surface electron affinity at the (2\ifmmode\times\else\texttimes\fi{}1)\ensuremath{\rightarrow}c(2\ifmmode\times\else\texttimes\fi{}2) transition is consistent with this model and results from a dipole induced by the charge transfer required to empty and fill all cation and anion dangling bonds on the c(2\ifmmode\times\else\texttimes\fi{}2) surface, respectively.