An investigation into the predictability of molecular adduct formation is presented by using the approach of hydrogen bond propensity. Along with the predictions, crystallisation reactions (1a–1j) were carried out between the anti-malarial drug pyrimethamine (1) and the acids oxalic (a), malonic (b), acetylenedicarboxylic (c), adipic (d), pimelic (e), suberic (f), azelaic acids (g), as well as hexachlorobenzene (h), 1,4-diiodobenzene (i), and 1,4-diiodotetrafluorobenzene (j); seven (1a to 1g) of these successfully formed salts. Five of these seven salts were found to be either hydrated or solvated. Hydrogen bond propensity calculations predict that hydrogen bonds between 1 and acids a–g are more likely to form rather than the H bonds involved in self-association, providing a rationale for the observation of the seven new salts. In contrast, propensity of hydrogen bonds between 1 and h–j is much smaller as compared to other bonds predicted for self-association/solvate formation, in agreement with the observed unsuccessful reactions.