We study the influence of an applied electric field on the fully resolved electronic spectra of aniline (AN), benzonitrile (BN), and 4-aminobenzonitrile (ABN) in the gas phase. Using these data, we test the commonly held but rarely proven assumption that the total dipole moment of a polyatomic molecule is the vector sum of bond dipole moments, localized in different parts of the molecule. We find that mu(a) (ABN) approximate to mu(a) (AN) + mu(a) (BN) in the excited S-1 state, but not in the ground S-0 state. Possible reasons for this non-additivity are discussed.