The ability to reproducibly synthesize nanocrystal (NC) inks with precisely controlled compositions is essential for making efficient kesterite solar cells from NCs. Here we present the results of a study on Cu–Zn–Sn–S NCs in which different particle size fractions were collected over a range of reaction times from various starting reagents. From this we have determined the temporal evolution of the NC ink and identified at least two distinct particle populations that form following injection: large particles containing primarily Cu and Zn, and small particles of Cu and Sn. For short reaction times, the extreme compositional heterogeneity between these size fractions makes the average ink composition highly sensitive to changes in reaction time and precipitation procedure. Longer synthesis times produce more consistent inks, with higher yield, and compositions closer to that of the starting reagents. The choice of metal precursor was found to have a minor impact on the composition of the resulting ink compared to the changes with time, even when substituting Ge precursors for Sn precursors. Using this understanding, we demonstrate the ability to produce inks with targeted off-stoichiometric compositions.