We report the first systematic experimental and theoretical study of the relationship between the linker functionalization and the thermodynamic stability of metal-organic frameworks (MOFs) using a model set of eight isostructural zeolitic imidazolate frameworks (ZIFs) based on 2-substituted imidazolate linkers. The frameworks exhibit a significant (30 kJ·mol-1) variation in the enthalpy of formation depending on the choice of substituent, which is accompanied by only a small change in molar volume. These energetics were readily reproduced by density functional theory (DFT) calculations. We show that these variations in the enthalpy of MOF formation are in linear correlation to the readily accessible properties of the linker substituent, such as the Hammett σ-constant or electrostatic surface potential. These results provide the first quantifiable relationship between the MOF thermodynamics and the linker structure, suggesting a route to design and tune MOF stability.