After the macroscopic pK a values and all predictions were made public, molecules SM07 and SM14 were analyzed in an NMR experiment to determine microscopic pK a s . SM14 has three nitrogens our algorithm identified as ionizable (Figure ). The NMR results indicated the two macroscopic pK a values were dominated by two different microscopic transitions included in our predictions (Table ). SM07 is a 4-amino quinazoline derivative with three nitrogens our algorithm identified as ionizable (Figure ). The NMR results indicated the macroscopic pK a was dominated by a sin- SAMPL6 included five other molecules that are also 4amino quinazoline derivatives. If we assume all of these molecules have a dominant microscopic transition on the same nitrogen, then we can compare a total of six predicted microscopic pK a s with experiment (Table 1). Our predictions for these microscopic transitions have reasonable correlation with the experimental values with an R 2 of 0.96±0.09 (Figure ). While there appears to be a slight bias with a mean error of 0.7±0.1, all predictions were within uncertainty of experiment. Predicted uncertainties are also fairly large, greater than 1.2 for all 4-amino quinazoline derivatives. High uncertainties are expected as our training data only included molecules with one or two ionizable groups and SM14 as well as these 4-amino quinazoline derivatives would not be well represented.