The effect of the adsorption and reaction of HCl and N(C2H5)(3) on the second harmonic generation (SHG) of Langmuir-Blodgett (LB) films made of nonlinear optical materials was studied. It was found that introduction of HCl or N(C2H5)(3) gases separately greatly reduced the SHG of the LB films, This is attributed as due to the modification of the film molecules by the acid or base. However, when the two of gases subsequently adsorbed, the SHG signal recovered, and in some cases it was several fold of magnitude higher than that from the original film. Both UV-vis spectroscopy and image of scanning electron microscopy supported the idea that a thin uniform layer of salt was formed on the matrix of the LB film molecules.