Several samples of magnesium fluoro-germanate doped with Mn 4+ were prepared by solid state synthesis with a different water vapor content in the synthesis atmosphere. A significant decrease in quantum yield and emission intensity was observed with the increase in water vapor content during synthesis. Although there was little effect on the emission spectra, the changes in excitation spectra indicate the dominance of magnesium germanate instead of magnesium fluoro-germanate at high water vapor content. X-ray diffraction (XRD) results show that the pure phase was only achieved with the least amount of water vapor. Refinement of XRD data estimates the quantities of different phases present. The effect on the grain size and morphology is also remarkable, as demonstrated by scanning electron microscopy imaging. The reason behind the changes is discussed in detail. The temperature dependence of the photoluminescence spectra was measured between room temperature and 500° C where the luminescence is quenched to nearly 3% of its initial value.