CHEMEST is an on-line system for estimating physical/chemical properties important in the assessment of environmental fate of chemicals. Measured values of boiling point, melting point, vapor pressure, water solubility, and soil organic carbon partition coefficient (Koc) were compared to values estimated by CHEMEST, for more than 170 chemicals. The chemicals were composed of seven classes: alcohols, aldehydes and ketones, amines, carbamates, esters, ethers, and phenols. Mean errors for boiling and melting point were 22.6 and 36.2 degrees C, respectively, with relatively little variability among classes. Carbamates were an exception, accuracy being much lower for both properties. Vapor pressures were quite accurately estimated, measured and estimated values generally differing by less than a factor of two. Mean error factors for water solubility and Koc were somewhat higher, but nevertheless better than the order of magnitude accuracy commonly assumed. Solubility was very inaccurately estimated for the phenols, however. The results indicate that, for the chemicals and properties studied, estimated values were sufficiently accurate for screening-level fate assessment. TSTCHEM, a revised version of CHEMEST, out-performed the latter with respect to the accuracy of solubility and vapor pressure estimates.