Technical Abstract: This study examined the influences of water chemistry on the quality of process water used in immersion chillers. During commercial poultry processing the bird carcasses come in direct contact with process water during washing and chilling operations. Contamination of the process water with bacteria from the carcasses occurs by design in bird washers where the objective is to remove the potential pathogens from the poultry skin and tissue surfaces. In contrast, immersion chillers use process water as the heat transfer fluid to rapidly reduce the temperature of bird carcasses. Contamination of the chiller water is a consequence of residual bacteria on the carcasses and subsequent distribution of the bacteria through the process water onto incoming carcasses. This is addressed by adding chemical sanitizers to the chiller water. A detailed investigation of the chemical interactions between a commercial sanitizer, trisodium phosphate, and the hard water ions of calcium and magnesium that are found in process water was performed. The speciation program Visual Minteq (J.P. Gustafsson, Department of Land and Water Resources Engineering, Stockholm, Sweden) was used to calculate pH, ionic strength, solubility, and concentration changes of process water at 8, 10, and 12 % treatment levels at temperatures of 0, 5, 25, 50°C. These results confirmed that the presence of hard water ions reduced the solubility of the sanitizer when present at high levels. Solubility also decreased as the temperature was reduced. These trends decrease the efficacy of sanitizer in the immersion chiller water with an expected increase in biological oxygen demand and reduction in process water quality.