The need to control toxicity resulting from wastewater chlorination is reviewed, and means for minimizing the problem through process optimization of the chlorination process is best achieved by providing a rapid initial mix, plug flow hydraulics in the chlorine contact chamber, and maintaining precise measurement and control of the chlorine residual. Dechlorination may be warranted in discharge areas of critical biological significance. The indirect and modified procedures of the iodometric method for chlorine residual determination are compared using wastewaters of various qualities. The data indicate that the indirect procedure with an amperometric titrator for endpoint detection should be used for accurately determining chlorine residuals in wastewaters.