Spinelloid phases have been observed and characterized by powder X-ray diffraction and high-resolution electron microscopy. Mg₃Ga₂GeO₃(III), with a narrow composition range of approximately 3 mole percent Mg₂GeO₄, is stable at atmospheric pressure up to about 1,420° C, and is isostructural with -Mg₂SiO₄ and the spinelloid Phase III of the NiAl₂O₄-Ni₂SiO₄ system. This represents the first occurrence of a -phase structure stable at 1 atm pressure. Above 1,420° C (1 atm) Mg₃Ga₂GeO₈ (III) decomposes reversibly into a mixture of spinel and olivine. At high pressure (around 30 kbar at 1,100° C) it transforms into another spinelloid phase, Mg₃Ga₂GeO₈ (IV), isostructural with Phase IV of the NiAl₂O₄-Ni₂SiO₄ system. In terms of crystal structures and phase relations therefore there exists a close analogy between the magnesium gallo-germanate and nickel alumino-silicate systems, the former being a lower-pressure analogue of the latter. Our investigation of a number of other pseudo-binary spinel-olivine oxide systems suggests that the formation of spinelloid phases can be associated with the inverse character of the spinel component.