Flowback water from natural gas extraction in Marcellus Shale contains very high concentrations of inorganic salts and organic chemicals. Potential reuse of this water in subsequent hydraulic-fracturing operations may be limited by high concentrations of divalent cations (e.g., Ba, Sr, and Ca). Kinetics of barite and celestite precipitation in flowback waters from different well sites was evaluated in this study. Ba reacted rapidly with sulfate and reached equilibrium within 30 min, whereas Sr reacted slowly and took days to reach equilibrium. Equilibrium concentrations of Ba and Sr predicted by thermodynamics models were compared with experimental results. Activity corrections based on the Pitzer equation provided the best agreement with experimental data for both Ba and Sr. Comparison of barite and celestite precipitation kinetics in actual and synthetic flowback water revealed that there was no observable impact of organics and other minor components in actual flowback water on barite precipitation rate. This was primarily due to the fact that barite precipitation occurred relatively quickly at the high saturation levels utilized in this study. By contrast, lattice poisoning and complexation with organic matter had a profound impact on the comparatively slower celestite precipitation. The presence of organic matter in actual flowback water increased Ba and Sr concentrations in solution, and contributed to the discrepancy between measured and predicted equilibrium concentrations.