We report an experimental study of a group of InAs-GaSb quantum wells which show finite ground subband energy splitting even in the absence of an external magnetic field. The origin of this zero-field splitting is identified as due primarily to the surface electric field mediated spin-orbit interaction. The InAs bulk structural asymmetry effects are ruled out by studying the QW thickness dependence of the splitting. We also determined the spin splitting as a function of magnetic field up to 15 T by using the tilted field technique. © 1990.