<![CDATA[Biodiesel’s cold flow properties, limited by the crystallisation of saturated and trans‑unsaturated fatty acid methyl esters (FAMEs), can be improved through skeletal isomerisation of unsaturated FAMEs to branched‑chain isomers. Zeolite‑β has shown promise for this reaction, but detailed characterisation of the complex isomer mixture remains challenging. This study investigates the isomerisation of methyl oleate over Zeolite‑β (H‑β) in a batch reactor at 200 °C and atmospheric pressure for 6 h, using 10 wt% catalyst. A conversion of 92.2% was achieved, confirming high catalyst activity. However, analysis by gas chromatography–mass spectrometry (GC–MS) with a polar BPX70 column revealed significant limitations: the product chromatogram showed extensive peak overlap, and the mass spectra of positional, geometric, and branched‑chain isomers were nearly identical, preventing reliable determination of selectivity and yield. The work highlights that conventional GC–MS is inadequate for full isomer speciation in such complex mixtures. Advanced techniques such as comprehensive two‑dimensional GC (GC×GC), silver‑ion chromatography, or high‑resolution mass spectrometry are required for accurate isomer identification. These findings establish a performance baseline for Zeolite‑β and delineate the analytical challenges that must be addressed to enable rational catalyst and process optimisation for improved biodiesel cold flow properties. Copyright © 2027 by Authors, Published by BCREC Publishing Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).]]>
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