The development of monosize superparamagnetic nanoparticles (NPs) with high saturation mass magnetization (Ms) and colloidal stability are required for advanced biomedical as well as electronic applications. Here we report synthesis, phase formation, morphology, magnetic and colloidal stability studies of monodisperse, single crystallined Zn1−xCoxFe2O4 (x = 0, 0.1, 0.3, 0.5 and 0.7) superparamagnetic NPs. Monodisperse NPs were synthesized by solvothermal reflux method through homogeneous nucleation and self focused growth protocols using quickly decomposable organic metal complex precursors and organic surfactant. All synthesized compounds show cubic mixed spinel structure with monodisperse spherical morphology. X-ray diffraction, electron diffraction, FTIR and Raman spectra analysis reveals that the ferrites are single phase compounds. However, the cubic lattice parameter reduces with enhance of Co2+ concentration due to small ionic radius compared to Zn2+. The Zeta potential studies shows that NPs form a stable dispersion in a nonpolar solvent at room temperature. The Ms enhances from 50.4 to 61.2 emu/g with substituent Co2+ concentration x = 0 to x = 0.7 in compounds. Enhanced magnetic properties may be useful in the fabrication of self assembled 2D layered electronic devices and biomedical applications such as drug delivery, magnetic contrast agents, magnetic hyperthermia materials. © 2017, Springer Science+Business Media, LLC.