Organic semiconductors are an emerging class of materials which, due to their ability to form thin films, have potential applications in the next-generation consumer electronics industry, including as flexible devices, transparent displays and organic LEDs. Their adoption is obstructed by their low charge mobilities: as much as 100 times lower than that of inorganic materials such as silicon and gallium arsenide. Charge mobility in organic semiconductors is limited primarily by thermal disorder and electron-phonon coupling. In many materials of this type, a small number of “killer” phonon modes dominate in scattering charges and therefore limiting electron mobility. Evaluating these phonon modes is key to informing the design of new organic semiconducting materials with superior charge transport properties, for example by adding high-mass side chains which may suppress key molecular frequencies. In this work, we present a combined method of electro-modulation terahertz spectroscopy on thin films with terahertz transmission spectroscopy on pellets to quantify the effects of thermal disorder on charge mobility in organic semiconductor materials, including C8-BTBT-C8 and TM-TES. The change in terahertz signal of a C8-BTBT-C8 device was measured as the amplitude of the applied square-wave bias was increased, and the mobility of the charge carriers was calculated to be 10.9±0.9 cm²/Vs. This is in accordance with literature values for the mobility of C8-BTBT-C8, demonstrating that this method is sufficient to accurately measure charge mobility in organic semiconductor devices, despite the very small change in signal which is measured. THz absorbance spectra for several organic semiconductors are presented.