Segmented microfluidics was applied to the ultrafast crystallization of dicarboxylate based MIL-88B type metal-organic frameworks (MOFs; Fe-MIL-88B-NH2, Fe-MIL-88B, and Fe-MIL-88B-Br). Particular attention was paid to the influence of the temperature, residence time, and slug volume on the size and crystal size distribution of the MOFs. Average sizes in the 90-900 nm range with relatively narrow crystal size distributions were obtained with residence times as short as 20 s depending on the MOF type and synthesis conditions.