We report that ethanol, used together with water, plays a crucial role in tuning the structures of a zirconium-based metal–organic framework and the 12-connected MOF-801, as well as the possible mechanisms of this modulating effect. By employing a cosolvent system of ethanol and water at just under room temperature without the presence of a monotopic carboxylic acid as the modulator, MOF-801 in various morphologies of different sizes could be synthesized. A linear correlation between the

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... /water ratio and the crystal sizes is also demonstrated. The growth mechanism is mainly explained by ethanol's bonding with the metal ion clusters and the Marangoni flow effect. Ethanol competes with the linker molecules in coordinating with the Zr metal clusters, a role similar to that of the modulators. The Marangoni flow effect, which dominates at a certain solvent ratio, further promotes the 1D alignment of the MOF-801 crystals.