New double complex bismuth salts of the general formula Cs2K[Bi(SCN)(x)I-6-x] where x = 1, 2 and Cs2K[Bi-2(SCN)(y)I-9-y] where y = 5, 6, 7 have been obtained by dissolving Cs[Bi(SCN)(4)] in KI solutions of various concentrations. The mechanism of the thermal decomposition of these compounds has been determined on the basis of thermal curves, chemical and diffractometric analyses of the solid pyrolysis products. The decomposition was found to proceed in three stages.