Size tuning for MoO2 nanoparticles is demonstrated for the first time over a wide range 2-100 nm, through a colloidal route into water. A nucleation-growth mechanism based on oriented attachment is evidenced to rationalize the impact of two simple synthetic levers: reactant ratio and temperature. The smallest non-aggregated crystalline MoO2 nanoparticles are reported, with specific surface area reaching 86 m(2) g(-1). Size and morphology control, along with the ability to produce, non-aggregated ultra-small MoO2 particles are important for a wide range of applications, such as catalysis and energy storage. To exemplify the importance of size tuning, the impact of downscaling on the electrochemical properties in Li-ion batteries is investigated.