The electro- optic responses of the stilbazium salt LB films were studied with differential surface plasmon spectrascopy in attenuation total reflection (ATR) geometry at three wavelengths (488 nm, 633 nm and 832 nm). It shows that the electro-optic differential ATR technique should provide for a widely applicable screening method for second-order nonlinearities in very thin films without recourse to a high power laser.