We present an optical "enantio-purification switch" that turns in two steps a racemic mixture of left-handed and right-handed chiral molecules into a pure sample containing the enantiomer of interest. The optical switch is composed of an "enantio discriminator" and an "enantio converter" acting in tandem. The method is robust, insensitive to decay processes, and does not require molecular preorientation. We demonstrate the method for the nanosecond purification of a racemate of (transiently chiral) D2S2 molecules. The energies of the rovibrational states and the related dipole elements are obtained by ab initio calculations. (C) 2003 American Institute of Physics.