Composition space diagrams have been used to study phase stability of mixed transition metal oxide fluorides synthesized by hydrothermal reaction of the metal oxides in (HF)(x)().pyridine/H(2)O/pyridine solution (150 degrees C, autogenous pressure). The combination of early (Ti, Zr, Hf, Nb, Ta, Mo, W) and late (Cu, Cd, Zn) transition metals and the effects of varying the mole ratios of the metals, (HF)(x)().pyridine, and water are examined in this study. Single-crystal products containing [Cu(py)(4)](2+), [Cd(py)(4)](2+), or [Zn(py)(4)](2+) cations and [TiF(6)](2)(-), [ZrF(6)](2)(-), [HfF(6)](2)(-), [NbOF(5)](2)(-), [TaOF(5)](2)(-), [MoO(2)F(4)](2)(-), or [WO(2)F(4)](2)(-) anions are recovered. Relative stability of the crystalline products is governed by the negative charge distribution on the anions, as well as the concentrations of each reactant in solution. Two new structures are reported: Cu(NC(5)H(5))(4)TaOF(5) and Cu(NC(5)H(5))(4)TiF(6).3H(2)O. Crystal data: for Cu(NC(5)H(5))(4)TaOF(5), monoclinic, space group C2/c (No. 15), with a = 10.541(3) Å, b = 13.547(6) Å, c = 16.04(1) Å, beta = 97.73(5) degrees, and Z = 4; for Cu(NC(5)H(5))(4)TiF(6).3H(2)O, monoclinic, space group C2/c (No. 15), with a = 12.767(4) Å, b = 12.048(2) Å, c = 17.787(6) Å, beta = 109.9(1) degrees, and Z = 4.