Pyrolysis of biomass is a means to industrially manufacture renewable oil and gas, in addition to biochar for soil amendment and long-term carbon fixation. In this work, oil and char derived from the slow pyrolysis of the unicellular marine diatom Tetraselmis chui are analysed using a variety of techniques. The pyrolytic oil fraction exhibits a wide variety of fatty acids, alkanes, alkenes, amides, aldehydes, terpenes, pyrrolidinines, phytol and phenols, with a high heating value (HHV) of 28 MJ/kg. The biochar produced has a HHV of 14.5 MJ/kg and reveals a number of properties that are potentially valuable from an agronomic point of view, including high cation exchange capacity (CEC), large concentration of N, and a low C:N ratio. The quantity of C in T. chui biochar that can be expected to stabilise in soil amounts to approximately 9%/wt of the original feedstock, leading to a potential net reduction in atmospheric CO(2).