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Organosilicon compounds are important in chemistry and commerce, and nearly 10% of new chemical substances for which premanufacture notifications are processed by the US Environmental Protection Agency (USEPA) contain silicon (Si). Yet, remarkably few measured values are submitted for key physical properties, and the accuracy of estimation programs such as the Estimation Programs Interface (EPI) Suite and the SPARC Performs Automated Reasoning in Chemistry (SPARC) system is largely unknown. To address this issue, the authors developed an extensive database of measured property values for organic compounds containing Si and evaluated the performance of no-cost estimation programs for several properties of importance in environmental assessment. These included melting point (mp), boiling point (bp), vapor pressure (vp), water solubility, n-octanol/water partition coefficient (logK(OW)), and Henry's law constant. For bp and the larger of 2 vp datasets, SPARC, MPBPWIN, and the USEPA's Toxicity Estimation Software Tool (TEST) had similar accuracy. For log K-OW and water solubility, the authors tested 11 and 6 no-cost estimators, respectively. The best performers were Molinspiration and WSKOWWIN, respectively. The TEST's consensus mp method outperformed that of MPBPWIN by a considerable margin. Generally, the best programs estimated the listed properties of diverse organosilicon compounds with accuracy sufficient for chemical screening. The results also highlight areas where improvement is most needed. Environ Toxicol Chem 2013;32:2433-2440. (c) 2013 SETAC