In order to pinpoint sources of error in log D predictions, separate log P and pK a challenges were organized for SAMPL6 . Better prediction performance was seen in the SAMPL6 octanol-water log P challenge compared to the SAMPL5 cyclohexane-water log D challenge. Performance improved in SAMPL6 for several reasons. First, the latter challenge avoided the pK a prediction problem. Second, far more experimental training data was available (aiding empirical and implicit QM methods).