We report measurements on the electrooptic response of thin samples (similar to 2-5 mu m) of two antiferroelectric liquid crystals. All the phase transitions in these compounds can be very easily detected using this technique. We have been able to measure such an electrooptic effect for the first time in the antiferroelectric and smectic I* phases of a tolane compound. The response shows a relaxation at high frequencies (similar to 10 KHz) and is attributed to a soft mode which produces an asymmetry in the molecular tilt in successive layers.