This investigation report examines a vapor cloud deflagration and fire that occurred on January 13, 2003, at BLSR Operating, Ltd., near Rosharon, Texas. The fire was caused by the release of hydrocarbon vapor during the unloading of basic sediment and water from two vacuum trucks into an open area collection pit. This report identifies the root and contributing causes of the incident. It makes recommendations on recognizing the flammability hazards of exploration and production waste liquids, and on safely handling flammable liquids. 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."