UV-visible absorption spectroscopy and second-harmonic generation were used to examine the effects of potassium iodide (KI) in the subphase on the optical and structural properties of pure and mixed hemicyanine derivative (DAEP) monolayers. It was found that with increasing KI concentration in the subphase, an enhancement of the effective molecular hyperpolarizability beta was always associated with a red shift of the absorption peak in both pure and mixed DAEP monolayers. Three mechanisms ("spacer" effect, charge distribution change and structural improvement) were responsible for the observed beta enhancement. The first two contributions were comparable and explained by a 2 omega resonance effect.