We have recently proposed an energy density analysis (EDA) that partitions the total energy of a molecular system into atomic energy densities. In this study, the EDA is applied to cis, trans-enol isomerization reactions of malonaldehyde, tropolone and 9-hydroxyphenalenone. Energy density changes in the reactions are shown to be closely related to the formation and breaking of the chemical bonds. By analyzing the energy density changes, we can find the reason why the hydrogen atom moves through the out-of-plane pathway instead of the in-plane pathway. (C) 2002 Elsevier Science B.V. All rights reserved.