We synthesised large (~2 kg) quantities of two chars for use as commercially available reference materials for the quantification of black carbon (BC). We pyrolysed chestnut wood (Castanea sativa) and grass straw (Oryza sativa) at 450 °C under a N2 atmosphere, which mimics the oxygen-free conditions on the inside of burning material at a moderate burning temperature. The charred materials were dominated by aromatic carbon (~70%), had low H/C (~0.7) and O/C (~0.3) ratios and low surface areas (2–6 m² g⁻¹). Isotopic changes on charring were small (≤0.3‰). In these respects, the synthesised chars were similar to chars produced at low temperature (<500 °C) in natural fires and thus may prove to be appropriate materials for calibrating BC quantification methods. Both chars have been used in a comparative study of BC quantification.