Predicting the solubility and stability of redox-active organics across all states of charge is a challenging task. A trained chemist can often predict trends, but when it comes to numerical values, even the best computer software cannot be counted upon. In screening active materials for redox flow batteries (RFBs) – in which both properties are important – we are slave to trial and error, modifying organic molecules in attempt to optimize (increase) solubility and stability without compromising other important properties such as redox potential.1,2We hope to change our approach to materials development by better predicting properties in advance. Using quantitative structure-property relationship (QSPR), we have shown that properties of unknown organic molecules can be predicted when experimental properties of a training set of related compounds is provided.3Here we sought to expand this approach to a different redox core, phenothiazines, which we have evaluated as posolyte candidates for RFBs. We measured the solubility of about a dozen phenothiazine derivatives – in both relevant states of charge (neutral and radical cation) – in a nonaqueous electrolyte. The structure of these derivatives included variations in the number, type, and position of substituents to offer a diverse training set. We used our experimental values and the results of density functional theory calculations to develop a QSPR model for the phenothiazine core, following which we synthesized and measured the solubility of a new set of phenothiazines. Here we will report on the degree of success of our model for solubility prediction. References Kowalski, J. A.; Casselman, M. D.; Kaur, A. P.; Milshtein, J. D.; Elliott, C. F.; Modekrutti, S.; Attanayake, N. H.; Zhang, N.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A., “A Stable Two-Electron-Donating Phenothiazine for Application in Nonaqueous Redox Flow Batteries.” J. Mater. Chem. A 2017, 5, 24371.Milshtein, J. D.; Kaur, A. P.; Casselman, M. D.; Kowalski, J. A.; Modekrutti, S.; Zhang, P. L.; Attanayake, N. H.; Elliott, C. F.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A., “High Current Density, Long Duration Cycling of Soluble Organic Active Species for Non-Aqueous Redox Flow Batteries.” Energy Environ. Sci. 2016, 9, 3531.Sevov, C. S.; Hickey, D. P.; Cook, M. E.; Robinson, S. G.; Barnett, S.; Minteer, S. D.*; Sigman, M. S.; Sanford, M. S. "Physical Organic Approach to Persistent, Cyclable, Low-Potential Electrolytes for Flow Battery Applications." J. Am. Chem. Soc. 2017, 139, 2924.