The terahertz Kerr effect (TKE) of polar molecular vapors is reported. The birefringence signal of fluoroform appears with opposite polarity compared to acetonitrile and water. This behavior is a hallmark of the opposite sign of a new molecular polarizability anisotropy [Formel], with [Formel] being the polarizability along the permanent dipole moment. As the excitation of the rotational states orients the permanent dipoles along the terahertz electric field, the orientation is translated into an optical birefringence proportional to [Formel]. Thus, the sign of [Formel] is imprinted onto the TKE signal, providing novel insights into the polarizability tensor of water.
