Supercritical antisolvent precipitation (SAS) is based on the fast dissolution of a liquid solution in a supercritical fluid. This technique has been tested to produce nanoparticles of yttrium, samarium, and neodymium acetates to be used as precursors of high-temperature superconductors. Particles of various morphologies were produced at different expansion levels of the liquid solution. Nanoparticles down to about 100 nm were obtained at very large expansion levels, whereas very large aggregates (balloons) with the diameter of several microns were produced at intermediate expansion levels. The influence of several process parameters on particle diameter and particle size distribution was studied. Among SAS process parameters, pressure, temperature, concentration of the liquid solution, and different liquid solvents were tested.