The decomposition of two peroxovanadate compounds, ammonium mu-oxo-bis(oxodiperoxovanadate(V)), (NH4)4[O{VO(O2)2}2], (NH4)4V2O11, and ammonium ammine-oxodiperoxo-vanadate(V), (NH4)[VO(O2)2(NH3)], has been investigated by means of X-ray powder diffraction, infrared and thermal analysis measurements. Above 100-degrees-C (NH4)4V2O11 is fully decomposed to (NH4)[VO(O2)2(NH3)] according to the reaction (NH4)4V2O11 reversible 2 (NH4)[VO(O2)2(NH3)] + H2O. Powder diffraction X-ray and infrared results show clearly that the forward reaction proceeds above 100-degrees-C and the backward reaction even at room temperature upon the addition of water to (NH4)[VO(O2)2(NH3)]. The latter compound decomposes to NH4VO3 at approximately 150-degrees-C. Upon standing at ambient conditions, a measure of decomposition of (NH4)4V2O11 also occurs, and because of this solid state reaction the mass loss found during the decomposition of "(NH4)4V2O11" can be lower than the calculated value.