This investigation report examines a chemical waste-mixing incident that occurred on April 25, 2002, at the Kaltech Industries Group, Inc., sign manufacturing facility in the Chelsea district of New York City. An explosion and fire in a confined basement workspace in a mixed-occupancy building injured several employees, members of the public, and six firefighters. This report identifies the root and contributing causes of the incident and makes recommendations on hazard communication, hazardous waste handling, and municipal oversight. The U.S. Chemical Safety and Hazard Investigation Board (CSB) is an independent Federal agency whose mission is to ensure the safety of workers, the public, and the environment by investigating and preventing chemical incidents. CSB is a scientific investigative organi- zation; it is not an enforcement or regulatory body. Established by the Clean Air Act Amendments of 1990, CSB is responsible for determining the root and contributing causes of accidents, issuing safety recommendations, studying chemical safety issues, and evaluating the effectiveness of other government agencies involved in chemical safety. No part of the conclusions, findings, or recommendations of CSB relating to any chemical incident may be admitted as evidence or used in any action or suit for damages arising out of any matter mentioned in an investigation report (see 42 U.S.C. 7412 r] 6] G]). CSB makes public its actions and decisions through investigation reports, summary reports, safety bulletins, safety recom- mendations, case studies, incident digests, special technical publica- tions, and statistical reviews. More information about CSB may be found at www.csb.gov."