The ab initio mol. dynamics method is extended to treat exchange of electrons between mols. and a reservoir at fixed chem. potential. The method is based on a rigorously grand-canonical d. functional approach using sep. potential energy surfaces for each oxidn. state. The resulting discontinuous dependency of excess charge on chem. potential is consistent with the statistical thermodn. of equil. gas-phase reactions. The method is illustrated by an application to the adiabatic redox dynamics of an aniline mol. [on SciFinder (R)]