Copper zinc tin selenide (CZTSe) nanocrystal inks show promise as a candidate for developing cheap, scalable, efficient, and nontoxic photovoltaic devices. They also present an important opportunity to controllably mix copper zinc tin sulfide (CZTS) with CZTSe to produce directly spectrally tunable Cu 2 ZnSn(S/Se) 4 (CZTSSe) solid-solutions using low-temperature processes. Herein, we describe a one-pot, low-temperature, aqueous-based synthesis that employs simultaneous redox and crystal formation reactions to yield CZTSe nanocrystal inks stabilized by Sn 2 Se 7 6- and thiourea. This versatile CZTSe synthesis is understood through the use of inductively coupled plasma mass spectrometry, Raman spectroscopy, Fourier transform infrared spectroscopy, and powder X-ray diffraction. It is further shown that stoichiometrically mixed CZTSe and CZTS nanocrystal powders yield a single CZTSSe phase at annealing temperatures between 200 and 250 °C. This facile and low-temperature process offers a low-energy alternative for the deposition of pure CZTSe/SSe thin films and enables the band gap to be readily tuned from 1.5 down to 1.0 eV by simple solution chemistry.