AbstractColloidal chemistry grants access to a wealth of materials through simple and mild reactions. However, even few elements can combine in a variety of stoichiometries and structures, potentially resulting in impurities or even wrong products. Similar issues have been long addressed in organic chemistry by using reaction-directing groups, that are added to a substrate to promote a specific product and are later removed. Inspired by such approach, we demonstrate the use of CsPbCl₃ perovskite nanocrystals to drive the phase-selective synthesis of two yet unexplored lead sulfochlorides: Pb₃S₂Cl₂ and Pb₄S₃Cl₂. When homogeneously nucleated in solution, lead sulfochlorides form Pb₃S₂Cl₂ nanocrystals. Conversely, the presence of CsPbCl₃ triggers the formation of Pb₄S₃Cl₂/CsPbCl₃ epitaxial heterostructures. The phase selectivity is guaranteed by the continuity of the cationic subnetwork across the interface, a condition not met in a hypothetical Pb₃S₂Cl₂/CsPbCl₃ heterostructure. The perovskite domain is then etched, delivering phase-pure Pb₄S₃Cl₂ nanocrystals that could not be synthesized directly.