When a discharger's effluent is found to be unacceptably toxic, regulatory agencies often require a Toxicity Reduction Evaluation (TRE) to be performed in an effort to eliminate or reduce the toxicity to acceptable levels. The major tool for TREs is a series of procedures termed a Toxicity Identification Evaluation (TIE). The objectives of the TIE procedures are to: (1) characterize the physical/chemical properties of the toxicant; (2) identify the specific toxicant or class of compounds causing toxicity; and (3) confirm whether the compound identified is causing the observed toxicity in the effluent. Presently, TIE procedures are useful in the characterization/identification of most types of toxicants. However, as a sample characteristic, total dissolved solids are difficult to address by existing TIE methodologies. For freshwater animals, TDS toxicity is a result of osmotic stress and its impact on the osmoregulatory capability of the organism. Through the use of case studies, this paper will address various procedures that have proven successful in quantifying the proportion of whole effluent toxicity that is due to TDS. Depending on the discharge situation, effluent toxicity due solely to TDS may be less of a regulatory problem, due to rapid dilution below toxic levels, and absence of human health or biomagnification concerns.