The high-frequency electro-optic tensor of succinonitrile is measured throughout the plastic crystalline phase. Its strength, due to cooperative molecular response to the orienting optical field, is a generalized susceptibility increasing rapidly near the plastic-solid transition. Its anisotropy describes long-range cubic order of the average molecular orientation. This orientational order parameter decreases rapidly near the plastic-liquid transition, suggesting that the hitherto neglected average orientational order is essential to the plastic phase stability. © 1974 The American Physical Society.