Developed about 15 years ago, sinter-bonding with Ag-nanoparticle (NP) pastes is now a well-established joining process, e.g. for die-attach in power electronics. More recently, also nanopastes based on Cu-NPs were introduced. However, Cu-NP oxidation during manufacturing and storage of the Cu-nanopastes can pose a considerable challenge. To avoid this issue, an alternative approach based on the in-situ reduction of Cu-oxide particles to metallic Cu-NPs by a reducing organic binder was proposed. To elucidate the reduction process and the sintering behaviour upon heating, we investigated the reaction of micro- and nanoscaled copper oxide particles with polypropylene glycol by use of synchrotron X-ray diffraction experiments. Combined with microstructural analysis by electron microscopy, in-situ synchrotron XRD allowed identifying the different reduction steps of Cu-oxide to Cu, deriving the corresponding reaction kinetics, and determining the development of the respective crystallite sizes during reaction and subsequent sintering. This in-depth study may serve as an exemplary case for in-situ XRD analysis of nanopaste sinter-bonding processes, and help to accelerate the development of novel nanopastes.