Terahertz (THz) radiation probes intermolecular interactions through crystal lattice vibrations, allowing the characterization of solid materials(1,2). Thus, THz spectroscopy is a promising alternative to mainstream solid-state analytical tools such as X-ray diffraction or thermal analysis(3). The method provides the benefits of online measurement(4), remote sampling(5) and three-dimensional imaging(6), all of which are attractive for quality control and security applications. In the context of pharmaceutical solids, THz spectroscopy can differentiate and quantify different forms of active pharmaceutical ingredients(7,8). Here, we apply this technique to monitor a dynamic process involving two molecular crystals(9). In particular, we follow the mechanochemical construction of a two-component cocrystal(10-12) by grinding together phenazine (phen) and mesaconic acid (mes)(13). To rationalize the observed changes in the spectra, we conduct lattice dynamics calculations that lead to the tentative assignment of at least one feature in the cocrystal THz spectrum.