Measurements of iodine speciation in aqueous solution at 285° C and 1000 psig show a higher percentage as I2 in solutions at trace concentrations than at higher concentrations. A 1 × 10−9 M I− solution resulted in 2% I2, whereas a 1 × 10−4 M I− solution had only 0.1% I2. These tests indicate that iodine at tracer concentrations can have a species distribution in solution that is not expected or observed at higher concentrations.