<p>This paper critically explores how ‘chemicals at the nanoscale’ are defined under the Australian Industrial Chemicals Introduction Scheme (AICIS) and compares it with the definitions established by the United States Environmental Protection Agency (US EPA) and European Commission (EC). Focusing on the properties of particle size and solubility, it explores how Australian regulators have translated the underlying nanoscience into their formal definitional framework. It then considers the regulatory implications that may arise from the application of Australia’s specific interpretation. </p><p>This study reveals significant differences in how particle size and solubility are applied within nanomaterial definitions. AICIS structures its definitional framework around these physical properties as regulatory proxies for material risk, relying heavily on archetypal behaviours and characteristics that may not apply universally across all substances. In contrast, American and European frameworks more explicitly recognize these criteria have limited predictive capability for risk assessment and have designed frameworks that allow regulators greater flexibility in identifying and assessing nanoscale chemicals. In application, AICIS’ more simplified approach creates a narrower regulatory scope that may poorly correspond with toxicological outcomes. This has direct implications for which substances are included or excluded from more stringent regulatory oversight. Future legal research should subsequently explore how AICIS’ definition is applied in practice, focusing on how regulators pursue enforcement actions for materials that challenge these regulatory parameters.</p>