The electrical properties of nanomaterials are strongly influenced by their surfaces, which in turn are strongly influenced by device processing and passivation procedures. Optical pump–terahertz probe spectroscopy is ideal for measuring the native properties of these materials, determining the changes induced by device processing, and studying the effectiveness of surface passivation procedures. Here we study the electronic properties of III–V nanowires and other nanomaterials in both their native and encapsulated/integrated states, which is uniquely possible with terahertz spectroscopy.