Abstract The ability of certain diatoms, notably genera of Haslea and Rhizosolenia , to biosynthesise a series of C 25 highly branched isoprenoid (HBI) alkenes or haslenes seems to have first evolved about 90 million years ago in the late Cretaceous. HBI alkenes and an alkane have been reported to occur in a wide variety of post-Cretaceous sediments and the alkane is used as a chemical fossil or biomarker to characterise some crude oils and sediments. Unlike the centric Rhizosolenid diatoms, the pennate diatom Haslea ostrearia biosynthesises HBIs principally by the methylerythritol phosphate (MEP) route. However, the structures of the intermediates in the MEP biosynthetic route to these important HBI geochemical biomarkers, or of any functionalised HBIs, have yet to be elucidated for H. ostrearia or indeed, other HBI producers. In the present study, a tri-unsaturated HBI alcohol and a di-unsaturated HBI epoxide have been isolated from a large scale culture of H. ostrearia . These compounds have been purified by column chromatography and HPLC and rigorously characterised using NMR spectroscopy and mass spectrometry. In addition, an authentic sample of the epoxide was synthesised and the spectra of the synthetic compound were indistinguishable from those of the isolated epoxide. The structures of other epoxides and alcohols are postulated on the basis of their mass spectral properties, though their low abundances prevented full structural characterisation with NMR methods. The structures of these new chemicals are discussed in relation to HBI biosynthesis and function. It seems likely that they may represent important overlooked intermediates between the MEP intermediate and the haslenes.