This document presents a harmonized method for characterising, for assessment purposes, a specific subcategory of UVCBs (Substances of Unknown or Variable composition, Complex reaction products or Biological materials): oleochemical substances. Many oleochemicals are UVCBs, due to the variability in the composition of the starting materials. The method presented in this document gives guidance on how oleochemical substances can be characterised in a way that their composition is accurately and consistently reflected to ensure that substances with the same chemical composition, manufactured in different countries, can be characterised with the same description for hazard assessment purposes. A common understanding and approach to characterising UVCBs would enable regulatory authorities to increase cooperation in the field of hazard assessment and help industry deal with regulatory requirements from multiple jurisdictions.