A spray pyrolysis approach has been used to obtain in-situ single-phase nanocrystalline Mn3O4 powders with spherically shaped particles. Ethanol or water-based solutions were sprayed at temperatures in the range of 450750 ÃÂ°C. Ethanol-based solutions led to carbon-contaminated powders at low temperatures and to particles with low porosity. Water-based solutions, however, are suitable for obtaining single-phase nanomaterials with a high specific surface area, up to 45 m2/g. The crystal size is less than 10 nm for spraying temperatures below 550 ÃÂ°C.