The Raman spectra of crystalline bis(pyrophosphato)tetrazinc(II) and -manganese(II) decahydrates, M4(P2O7)2 ·10H2O (M=Zn and Mn), have been investigated at room temperature. The Mn(II) salt readily underwent photodecomposition upon extended exposure to 785-nm laser irradiation and so could not be examined under high pressures in a diamond-anvil cell. The Zn(II) salt was much more stable and was studied successfully at pressures up to ∼35kbar. The pressure dependences of the δ sPO2, ν sPOP, ν asPOP, ν sPO2, and ν asPO2 internal modes of the pyrophosphate group and of an external H2O mode indicated the occurrence of a pressure-induced phase transition in the 15–20kbar region. Further confirmation of this structural change came from the appearance of four initially forbidden modes in the 19–26kbar range. This phase transition proved to be reversible upon release of the applied pressure. The pressure dependences before and after the phase transition were −0.22 to 0.66 and 0.06 to 0.33cm−1/kbar, respectively.