Effects of a gravity wave on the electron concentration in the upper mesophere including the temperature effects of the wave on chemical reaction rates, are studied analytically. To reproduce the ledge of the D region, a simple working model of the fast formation of H= (H2O)n is presented, in which NO+(H2O)2 in some excited state is hypothesized to be exothermically converted to H+(H2O)2. Using the model, we show that a wave, with periods longer than 2 hours, interacts remarkably with the ledge of the D region, where the amplitude of electron response reaches a level about 20 times as large as the response of the neutral density. The modulated ledge moves, with the propagation of the wave, gradually downward, while being replaced by a new ledge from above. The general agreement of the obtained results and observations suggests that the thermal decompoisition of NO+H2O⋅N2 has an important role in controlling the compositions of positive ion species and thereby the structures of electron profiles in the upper mesosphere. It is stressed that the irregular profiles of electrons observed in the upper mesosphere are caused, to a considerable extent, by the effects of gravity waves. ¿ American Geophysical Union 1988