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): hydrocarbon solvent substances. The method presented in this document gives guidance on how hydrocarbon solvent substances can be characterised in a way that their composition is accurately and consistently reflected. This ensures 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.