A molecular-level mechanism is proposed for two cases of mechanochemical cocrystallization via halogen bonds. The proposed stepwise mechanism is based on the observation and structural characterization of intermediates that appear in early stages of the reaction. The mechanism arises from the competition of strong and weak intermolecular halogen bonds of the N center dot center dot center dot I and S center dot center dot center dot I type and involves the initial formation of finite molecular assemblies, held together via N center dot center dot center dot I bonds that subsequently polymerize into infinite chains by cross-linking through S center dot center dot center dot I bonds. This mechanism has been observed in the synthesis of linear as well as zigzag halogen-bonded chain architectures.