A coalition of Swedish environmental groups called the International Chemical Secretariat (ChemSec) has developed a Web-based tool to help the lay public identify substances that are structurally similar to known hazardous substances. Compounds that are structurally similar often exhibit related hazardous properties. The tool, SINimilarity, compares chemicals with those on ChemSec’s Substitute It Now (SIN) List, a global database of chemicals that ChemSec considers “substances of very high concern” based on criteria under the European Union’s Registration, Evaluation, Authorisation & Restriction of Chemicals (REACH) program. The SIN List contains more than 800 chemicals. SINimilarity is intended to serve as a screening tool for investigating the safety of chemicals designed to replace toxic substances. Users can investigate nearly 500,000 chemicals by name, CAS Registry Number, or structure with SINimilarity to determine if a particular chemical is more or less toxic than the chemical it is intended to replace. The tool is at sinimilarity.chemsec.org .