Little is known concerning the precise molecular pathway that links fluid-phase molecules to those in nascent crystal nuclei. In this paper the process of molecular self-assembly has been studied in concentrated solutions using FTIR spectroscopy. Three carboxylic acids, benzoic, tetrolic, and mandelic acids, have been chosen on the basis of their differing crystal chemistries, as reflected in observed hydrogen-bonding motifs. Using the solid-state spectra as a means of unambiguous assignment of carboxyl and hydroxyl vibrations associated with hydrogen bonding, spectroscopic data are reported for solutions as a function of both composition and solvent. In the cases of benzoic acid and tetrolic acid, a link between the growth synthon and the structural synthon is apparent. Mandelic acid, on the other hand, provides a more complex case in which strong solvation effects are evident, leading to the conclusion that significant molecular rearrangement must occur within the developing crystal nuclei. © 2006 American Chemical Society.