The changes of 31 P nuclear magnetic resonance spectra of P2I4-CS2 solutions and of molecular weights of P2I4 in benzene with time have shown that in inert solvents a gradual disproportionation occurs with formation of PI3 . White Phosphorus, P4 , is not formed by this process. Instead the initially formed disproportionation product with higher Phosphorus content changes into a polymeric solid PnI by cleavage of PIS , which accounts for the increasing displacement of disproportionation equilibrium. The UV spectrum of P2I4 in n-Hexan is reported. The molar extinction coefficients indicate a considerable derealization of unshared electron pairs of the Phosphorus atoms.