Potassium niobate-lead titanate solid-solution nanoparticles have been synthesized using a new hydrothermal method. The key step is an in situ acid-base exothermic reaction with a large excess of KOH, generating an amorphous precipitated gel. This gel, when crystallized in an autoclave for 2h at 190 degrees C, produces a mixed perovskite solid solution with [KNbO3](0.8)-[PbTiO3](0.2) (KNPT) composition, and parallelepiped particles rang in size from a few tens to a few hundreds of nanometer in length. This study proves that the pure perovskite phase can be obtained in spite of the different solubilities of the reactive cations in an ethanol-water solution. Dielectric measurements show that KNPT ceramic with a grain size of 50nm is ferroelectric at room temperature. In addition, ceramics sintered at 950 degrees C have an average grain size of 500nm, and exhibit a maximum permittivity of 2100 at 1kHz near T-C=510K and a remanent polarization P-r=12C/cm(2