Due to special properties such as superparamagnetism, magnetic nanoparticles have important technological, environmental and medical applications. In this work, nanoparticles of magnesioferrite (MgFe2O4) were prepared by coprecipitation from chloride solutions of iron and magnesium. Compared with traditional procedures, this method has the advantages of low cost, simplicity and the use of moderate annealing temperatures. The properties of the nanoparticles were investigated using X-ray diffraction to determine the average size of the crystallographic domains and Mössbauer spectroscopy to find the particle size distribution. The results showed that the coprecipitation method yields magnesioferrite nanoparticles with a fairly narrow size distribution.