Among the procedures to prepare lanthanide-containing nanoparticles a gap exists in the range between 5 and 40 nm. The miniemulsion technique presented here is intended to fill this discontinuity and offers a facile method that can be applied for the preparation of nanoparticles for various applications, e.g. medical imaging, optics and catalysis. We demonstrate that formation of nanodroplets under emulsion conditions is the key step in the size control of the nanoparticles. The type of surfactant and the nature of the dispersed and continuous phases strongly influence the interfacial activity and, consequently, the size of the final solid particles that result from the subsequent thermal decomposition. Moreover, the choice of the surfactant determines the final elemental composition of the particles, leading to either lanthanide oxides or oxysulfates when using Brij® 35 or sodium dodecyl sulfate, respectively. Nanoparticles of holmium and gadolinium were prepared and their applicability as magnetic resonance imaging contrast agents is shown.