A study was conducted to demonstrate the fabrication of colloidal Mn xFe1-xO solid-solution nanocrystals and their application as a simultaneous and dual MRI contrast agent for T1- and T 2--weighted imaging. MnxFe1-xO solid-solution nanocrystals were synthesized by the thermal decomposition of iron acetylacetonate(III) and manganese acetylacetonate(II) precursors. Elemental analysis of the solid solution was performed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and the energy-dispersive X-ray spectroscopy. Transmission electron microscopy (TEM) investigation revealed 20 nm nanocrystals with a crystalline structure. MRI measurements were also performed on a clinical 1.5-T magnetic resonance scanner at room temperature for three different water-dispersible MnxFe1-xO nanocrystals.