Standard thermochemical data (in the form of Delta H-f(o) and Delta(f)G(o)) are available for crystalline (c) materials but rarely for their corresponding amorphous (a) counterparts. This paper establishes correlations between the sets of data for the two material forms (where known), which can then be used as a guideline for estimation of missing data. Accordingly, Delta H-f(o) (a)/kJ mol(-1) approximate to 0.993 Delta H-f(o)(c)/kJ mol(-1) + 12.52 (R-2 = 0.9999; n = 50) and Delta(f)G(o) /kJ mol(-1) approximate to 0.988 Delta H-f(o)(c)/kJ mol(-1) + 0.70 (R-2 = 0.9999; n = 10). Much more tentatively, we propose that S-298(o)(c)/J K-1 mol(-1) approximate to 1.084S(298)(o)(c)/J K-1 mol(-1) + 6.54 (R-2 = 0.9873; n = 11). An amorphous hydrate enthalpic version of the Difference Rule is also proposed (and tested) in the form [Delta H-f(o)(MpXq center dot nH(2)O,a) - Delta H-f(o)(MpXq,a)]/kJ mol(-1) approximate to Theta(Hf)n -302.0n, where MpXq center dot nH(2)O represents an amorphous hydrate and MpXq the corresponding amorphous anhydrous parent salt.