Single-crystal films of an organic molecular salt, -dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) with exceptional optical quality are prepared by a modification of the shear method. Electro-optic modulation for light propagating perpendicular to the film is measured at 720 and 750 nm wavelengths with electric fields over the frequency range of 2 kHz to 100 MHz. A modulation depth of 80% was measured at 750 nm for a field of 4 V/μm in a 4-μm-thick film. The measured electro-optic response is flat over the frequency range used (2 kHz to 100 MHz), indicating electronic origin of the electro-optic effect. The results show that these films may lead to a new generation of electro-optic devices. © 2003 Society of Photo-Optical Instrumentation Engineers.