A computationally-assisted methodology to assign the vibrational spectra of molecular materials (PiMM) has been successfully applied to the Raman and infrared (IR) spectra of trans-cinnamic acid (t-CIA) in the solid state. The effects of molecular association through O-H center dot center dot center dot O hydrogen-bonding in both the wavenumber and the intensity of the bands are accurately described by a single dimer structure. This supports the discrimination between the possible conformers in the solid, the complete evaluation of hydrogen-bonding effects, and the full assignment of the vibrational spectra of t-CIA.