Approximately 2.0 nm diameter nanocrystalline zinc sulfide was synthesized in water (H2O-ZnS) and -2.4 nm nanocrystalline mercaptoethanol-capped ZnS was synthesized in a mercaptoethanol-water solution (merZnS) in order to investigate the connections between the crystal growth pathway and microstructure development under hydrothermal conditions in water. XRD, HRTEM, and kinetic modeling were used to analyze reaction progress, determine the growth mechanism, and identify microstructural features. XRD and HRTEM data indicate that the H2O-ZnS particles often contain planar defects, but these are uncommon in the initial mercaptoethanol-capped samples. Kinetic modeling and HRTEM data indicate that early crystal growth of both H2O-ZnS and mercaptoethanol-capped ZnS occurs predominantly via crystallographically specific oriented attachment (OA). Twins and stacking faults form in the coarsened mercaptoethanol-capped ZnS, whereas more complex, closely spaced twins, stacking faults, and polytypic intergrowths are characteristic of coarsened H2O-ZnS. These defects are consistent with those predicted for OA-based growth involving the microstructurally distinct initial particles. At longer reaction times, diffusion-controlled growth removes surface irregularities arising from OA to yield rounded particles with complex internal structures.