By combining vibrational spectroscopy in the far-infrared region, measured by terahertz timedomain spectroscopy (THz-TDS), and ab initio DFT simulations under periodic boundary conditions, we seek here to obtain further insight into the nature of aspirin form I/II polymorphism. The measurement of vibrational dynamics complements the structural characterisation from crystallographic techniques, while the solid-state DFT methods provide a powerful route for interpretation of the terahertz spectra. These methodologies have previously been applied to study a wide range of organic molecular crystals, including polymorphs of pharmaceutical drug molecules. THz-TDS probes weak interactions within molecular crystals, but due to the complexity of the motions at terahertz frequencies, not least given the strong coupling between large amplitude external and internal motions, the spectra are difficult to interpret without the complementary DFT method. The terahertz spectrum of aspirin form I has previously been measured by Laman, Harsha and Grischkowsky using a bespoke THz-TDS setup, where thin films of crystals were grown directly onto the surface of an aluminium parallel plate waveguide structure. This method has the advantage that no scattering occurs when the terahertz electric field propagates along the surface of the waveguide and the effervescent field can interact very efficiently with the crystalline film. As a result, the line width of the vibrational modes is dramatically sharpened compared to conventional transmission spectroscopy of a polycrystalline sample (Figure ). However, the requirement to crystallise the sample directly onto the flat aluminium plate waveguides in the form of a thin film restricts the application of the method for detailed investigation of complex polymorphism where such constraints in crystallisation conditions limits the accessible solid-state forms. Therefore, we apply transmission measurements on pellets containing polycrystalline particles of the two polymorphic forms of aspirin using a commercial terahertz spectrometer.