The last 2 decades have seen the synthesis of a variety of polynuclear phosphorus compounds with phosphorus-phosphorus-bonded skeletal arrangements. However, many synthetic aspects of these compounds have remained unclear. The effect of solvent on the relative stability of the hexadecaphosphide dianion (P-16(2-)) and the henicosaphosphide trianion (P-21(3-)) is established in this account, thus demonstrating the first redox interconversion of two homopolyatomic phosphorus anions.