Phase pure Li3As and Li3P were synthesized from the elements by a high temperature route. Crystal structures were refined from powder X-ray diffraction data. The title compounds were further characterized by difference thermal analysis, temperature dependent X-ray powder diffraction and impedance spectroscopy, proving unexpected Li ion conductivity for Li3As. High pressure behaviour of the title compounds was modeled via density functional theory, confirming the experimentally reported cubic modifications of Li3P and Li3As.