Nanoparticle-based, solution-processed chalcopyrite photovoltaic devices have drawn tremendous attraction for the realization of low-cost, large-area solar cell applications. In particular, it has been recently demonstrated that the CuSe phase plays a critical role in allowing the formation of device-quality, nanoparticle-based chalcopyrite absorber layers. For further in-depth study, with the aim of understanding the thermal behavior of the CuSe phase that triggers the vigorous densification reaction, a requisite for high-performance chalcopyrite absorber layers, both multiphase (CuSe-phase including) and single-phase (CuSe-phase free) CISe nanoparticles are investigated from the viewpoint of compositional variation and crystalline structural evolution. In addition, with CuSe-phase including CISe particulate layers, the basic restrictions in thermal treatment necessary for activating effectively the CuSe-phase induced densification reaction are suggested, in conjunction with consideration on the thermal decomposition of organic additives that are inevitably incorporated in nanoparticle-based absorber layers.