By finely tuning the sizes of pores in metal-organic framework (MOF) compounds, chemists have designed solid sorbents that thoroughly separate branched paraffins from their linear analogs (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201506345). Sorbents that completely discriminate between compounds on the basis of small differences in molecular size could potentially lower the cost of separating light hydrocarbons for fuel production. That separation is typically done via fractional distillation, an energy-intensive process. Mohamed Eddaoudi of King Abdullah University of Science & Technology and colleagues had previously synthesized rare-earth-based MOFs that showed potential for separating branched alkanes from linear ones. Now, by shortening the organic linkers that join the metal clusters, the team has tuned the pore size and improved the separation efficiency. Specifically, by joining yttrium- and terbium-based building blocks with fumarate linkers, the team synthesized MOFs with 4.7-Å-wide pores. On the basis of various sorption tests, they showed that the new MOFs completely separate butane from isobutane and pentane from isopentane and predict similar results for isomers of longer paraffins.