We describe the oxonitridophosphates M(1-x)PO(3+4)xN(1-4x) (x approximate to 0.05) and the isotypic oxophosphates M0.75PO4 (M = Zr, Hf) obtained by high-pressure metathesis. The structures (ZrSiO4-type, space group I4(1)/amd (no. 141), a = 6.5335(7)-6.6178(12), c = 5.7699(7)-5.8409(9) angstrom, z = 4) were refined from single-crystal X-ray diffraction data, and the powder samples were examined with quantitative Rietveld refinement. Infrared spectroscopy did not indicate the presence of X-H (X = 0, N) bonds. The optical band gaps, between 3.5 and 4.3 eV, were estimated from UV-vis data using the Kubelka-Munk function under assumption of a direct band gap. Temperature-dependent powder X-ray diffraction showed a phase transformation of the M0.75PO4 (M = Zr, Hf) compounds to ambient pressure polymorphs at 780 (Zr) and 900 degrees C (Hf). The preparation of the nitrogen containing compounds exemplifies the systematic access to the new class of group 4 nitridophosphates granted by high-pressure metathesis. Moreover, we show that high-pressure metathesis can also be used for the preparation of metastable oxophosphates.