Substances of unknown or variable composition, complex reaction products or biological materials (UVCBs) have been conventionally described in generic terms. Commonly used substance identifiers are generic names of chemical classes, generic structural formulas, reaction steps, physical-chemical properties or spectral data. Lack of well-defined structural information has significantly restricted in silico fate and hazard assessment of UVCB substances. A methodology for structural description of UVCB substances has been developed that allows use of known identifiers for coding, generation and selection of representative constituents of UVCB substances. The developed formats Generic SMILES (G SMILES) and Generic Graph (G Graph) address the need to code, generate and select representative UVCB constituents. G SMILES is a SMILES-based single line notation coding fixed and variable structural features of UVCBs. G Graph is based on a workflow paradigm which allows generation of constituents coded in G SMILES and endpoint specific or non-specific selection of representative constituents. Structural description of UVCB substances as afforded by the developed methodology is essential for in silico fate and hazard assessment. Data gap filling approaches such as read across, trend analysis or (Q)SAR can be applied to the generated constituents and the results used to assess the substance as a whole. The methodology also advances the application of category based data gap filling approaches to UVCB substances.