The solvent extraction behavior of radioiodine at tracer concentration was studied by varying the pH of the tracer solution in order to gain deeper insight into the anomalous behavior of radioiodine at low concentrations. The anomalous behavior of radioiodine was observed with all the tracer solutions because of the existence of radioiodine complex in the tracer solution. The observed anomalous behavior of radioiodine was consistently explained by the comprehensive extraction mechanism.