The solid-state structure of a nonionic adduct of phosphorus(V) chloride and pyrazine, C4H4N2-PCI5, shows phosphorus to be six-coordinate with one octahedral position occupied by a nitrogen atom of the pyrazine ring. The monoclinic crystal is of space group C2/c having four molecules per unit cell of dimensions a = 8.359 (8) A, b = 16.202 (15) A, c = 7.568 (7) A, and p = 99.37 (7)'. The structure was refined by least squares to R = 0.097 by using 1754 independent reflections collected to 28, = 65" and corrected for absorption. The P-N distance is 2.021 (5) A. In nitromethane solution, Raman, 'H NMR, cryoscopic, and conductivity measurements are interpreted in terms of equilibria involving CIH4N2.PCIS, pyrazine, molecular PCI,, and ions originating from the ionization of PCls and C4H4N2.PClS.