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The Thermodynamic Difference Rules are capable of assessing the quality of a given set of thermodynamic data. This fact, and their inherent simplicity, makes them suitable for application by non-specialists. They are here applied to test existing literature values for the enthalpy of formation of the crystalline “hydrates” which are derived from the oxides of As(V) and P(V). The Δ f H ° data for the arsenic compounds are immediately validated. However, in the case of the phosphorus data, we conclude that the use of ½Δ f H °(P 4 O 10 ,s) to provide the parent oxide data is inappropriate. A value of Δ f H ° for P 2 O 5 , of −1607 ± 12 kJ mol −1 is now proposed. This is consistent with other recent estimations of Δ f H °(P 2 O 5 ,s). Similarly, values are proposed for Δ f G °(P 2 O 5 ,s)/kJ mol −1 ≈ −1514 ± 18 and S 298 °(P 2 O 5 ,s)/J mol −1 K −1 ≈ 110. Additionally, in cases where several values for a given thermodynamic property of the As(V) and P(V) oxide hydrates appear in the literature, these measurements are appraised in the context of the Difference Rules; in cases where no data are listed, values are now capable of being estimated. [ABSTRACT FROM AUTHOR]