Environmental effects on the electronic structure of molecules caused by external perturbations are studied by means of an expansion of molecular orbital wave functions into valence‐bond (VB) wave functions. Applications are carried out to the π system of butadiene, hydroxyethylene, acrolein, and carbonyl oxide and also to the full electronic system of water. The external perturbations considered are uniform electric fields, nonuniform electric fields, and reaction fields created by solvents. A different behavior is found between nonpolar and polar species in the presence of solvents. Some consequences on the behavior of molecules under the influence of the different electric fields are discussed. Copyright © 1991 John Wiley & Sons, Inc.