Rapid a.c. polarographic methods possess certain advantages in studies of complex ions of sparingly soluble systems involving reversible electrode processes. The sparingly soluble lead(II)-fluoride system, studied previously, was re-examined in detail by the rapid a.c. method; excellent agreement was obtained with earlier values. The mixed complex system lead(II)/3-chloride/3-fluoride was then evaluated. The complex PbClF was detected and a stability constant of 525 was obtained at an ionic strength of 1.0 and 25°. The general need for eliminating oxygen in a.c. polarography is discussed.