Abstract Semitransparent gold electrodes on capacitance samples and a crystal with optical ports have allowed the simulataneous observations of dielectric properties and birefringence in KDP type crystals. We have confirmed the previously reported temperature hysteresis of 1/e KDP and KDA. We have further observed the temperature of appearance and disappearance of the domain walls of the ferroelectric phase of these materials. In the ferrolectric phase we have measured P vs. E hysteresis loops using a DC method and photographed the strong edge effects in the capacitance samples wtih applied fields.