The Soret effect in an ionic crystal of the type MX doped with divalent impurity ions is considered from the point of view of lattice defects. On the assumption that the impurity ions move in the form of tightly bound impurity-vacancy complexes, the relative concentration gradient of impurity ions is expressed in terms of the heat of transport of the complex and in terms of quantities which may be determined from electrical conductivity measurements. Following the kinetic theory of Wirtz, the heat of transport of the complex is simply related to the heats of transport of the elementary jumps involved in the motion of the complex. An estimate is made of the magnitude of the Soret effect in AgBr containing CdBr2. It is found that the relative concentration gradient should be large enough to be measured experimentally.