Two stable nanofluids comprising of mixed valent copper(I,II) oxide clusters (<1 nm) suspended in 1-butyl-3-methylimidazolium acetate, [C₄mim][OAc], and copper(II) oxide nanoparticles (<50 nm) suspended in trioctyl(dodecyl)phosphonium acetate, [P_8 8 8 12][OAc], were synthesised in a facile one-pot reaction from solutions of copper(II) acetate hydrate in the corresponding ionic liquids. Formation of the nanostructures was studied using ¹³C NMR spectroscopy and differential scanning calorimetry (DSC). From a solution of Cu(OAc)₂ in 1-ethyl-3-methylimidazolium acetate, [C₂mim][OAc], crystals were obtained that revealed the structure of [C₂mim][Cu₃(OAc)₅(OH)₂(H₂O)]·H₂O, indicating the formation of copper hydroxo-clusters in the course of the reaction. Synthesised nanostructures were studied using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Physical properties of the prepared IL-nanofluids were examined using IR and UV-VIS spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and densitometry.