A description is given of the physical configuration, thermal characteristics, and operating limitations of the three ice prevention subsystems for counterflow natural draft towers. The thermal characteristics of the ice prevention subsystems for a typical 500 foot high counterflow tower are illustrated through the use of multivariate performance curves, generated by a highly empirical computer model developed by the authors. The usefulness of these theoretical temperature predictions lies primarily in the definition of performance trends and thermal interactions of the subsystems. The absolute values of temperature predictions of this type are, of course, dependent on tower geometry and the type of fill system used in the tower. The performance curves included correlations of up to four specific, interdependent subsystem water temperatures.