Solvents are often used to aid test item preparation in aquatic ecotoxicity experiments. This paper dis-cusses the practical, statistical and regulatory considerations. The selection of the appropriate control (ifa solvent is used) for statistical analysis is investigated using a database of 141 responses (endpoints)from 71 experiments. The advantages and disadvantages of basing the statistical analysis of treatmenteffects to the water control alone, solvent control alone, combined controls, or a conditional strategy ofcombining controls, when not statistically significantly different, are tested. The latter two approachesare shown to have distinct advantages. It is recommended that this approach continue to be the standardused for regulatory and research aquatic ecotoxicology studies. However, wherever technically feasiblea solvent should not be employed or at least the concentration minimized.