A surfactant-free synthesis of highly monodisperse Co3O4 nanoparticles via a sol-gel route was scaled-up in order to yield up to one gram of nanoparticles with a narrow size distribution. Parameters allowing for an accurate control of the crystallite size were identified and systematically evaluated. Application of Taguchi's method for experimental design revealed an increase in crystallite size with increasing synthesis temperature or concentration of the cobalt precursor. However, the amount of additive NH4OH(aq), respectively the associated water, has the most pronounced effect on the resulting crystallite size. Water was identified as the main source of oxygen and is required to yield single-phase Co3O4 nanoparticles. An ideal mixing of the reaction mixture is crucial for the availability of water during the synthesis. The outcome allows for the controlled synthesis of monodisperse, single-phase Co3O4 nanoparticles in the range of 2.8 to 9.8 nm with relative standard deviations ranging from 13 to 19%. Further, conducting the synthesis in the absence of water yields monodisperse, single-phase, and highly crystalline CoO nanoparticles. (C) 2018 Elsevier B.V. All rights reserved.