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Ethyl chloroformate (ClC(O)OC2H5), a room-temperature liquid, adopts two crystalline structures under different cryogenic conditions. Lattice cohesive energies indicate thatcontrary to a common notionthe high symmetry, low Z′ polymorph (R3̅, Z′ = 1; Form II) is a kinetically stable form compared to the low symmetry, high Z′ polymorph (P1̅, Z′ = 2, Form I). This also corroborates our observation of Form II showing a tendency to convert into the thermodynamically more stable Form I in the in situ cryo-crystallography experiments. Interaction topology analysis indicates that the primary packing motif in both polymorphs is formed with an antiparallel CO···CO dipolar interaction. Differences in secondary interactions, particularly those involving chlorine, drive the formation of polymorphs. The Cl atom is engaged in Cl···O halogen bonding in both polymorphs, however, with a striking difference in its mode of interaction. It chooses an ethoxy O atom (O–C2H5) as acceptor in Form I vs a carbonyl O atom (OC) as acceptor in Form II. Quantitative analysis of the intermolecular interactions in both these polymorphs in terms of CE-B3LYP interaction energies, electron density topology analysis using the Quantum Theory of Atoms in Molecules (QTAIM) approach, electron deformation densities, and electron localization function (ELF) provide insights into their nature and explain relative stability trends of these polymorphs.