Ultrasmall (9-30 nm) Yb3+/Er3+-doped, upconverting alkaline rare-earth fluorides that are promising for future applications were synthesized by the microwave -assisted hydrothermal method. The formation mechanism was proposed, indicating the influence of the stability of metal ions complexes with ethylenediaminetetraacetic acid on the composition of the product and tendency to form M2REF7 (M0.67RE0.33F2.33) cubic compounds in the M RE F systems. Their physicochemical properties (structure, morphology, and spectroscopic properties) are compared and discussed. The obtained nanoparticles exhibited emission of light in the visible spectra under excitation by 976 nm laser radiation. Excitation and emission spectra, luminescence decays, laser energy dependencies, and upconversion quantum yields were measured to determine the spectroscopic properties of prepared materials. The Yb3+/Er3+ pair of ions used as dopants was responsible for an intense yellowish -green emission. The upconversion quantum yields determined for the first time for M2REF7-based materials were 0.0192 +/- 0.001% and 0.0176 +/- 0.001% for Sr2LuF7:Yb3+,Er3+ and Ba2LuF7:Yb3+,Er3+ respectively, the two best emitting samples. These results indicated the prepared materials are good and promising alternatives for the most studied NaYF4:Yb3+,Er-3 nanoparticles.