Similar trends (poor performance in water, better performance in solvents where the represented acids are less diverse) were also observed using the COSMO-RS method; in fact, the relative ordering of which solvent predictions had higher error is nearly identical (see SI). COSMO-RS is driven by quantum mechanics including some parameterization on data; hence, errors from the COSMO-RS method tend to be based in physical limitations (e.g. more challenging quantum-chemical systems to model), whereas our ML model is purely data-driven. Therefore, it is somewhat unexpected that the errors would be so similar.