The Experimental Physical Chemistry Program of the Chemistry Division is supporting research by Professors De Lucia and Herbst in high-frequency microwave spectroscopy with emphasis on the collisional tranfer of rotational energy and the spectroscopy of molecular ions. Novel low-temperature line-broadening studies are underway to lead to increased understanding of the energy transfer mechanism. The techniques are also ideally suited to the investigation of the identity and structure of molecular ions. %%% When a beam of gas molecules is rapidly expanded, it cools to temperatures as low as 1K, giving an opportunity to study a number of spectroscopic properties uncomplicated by vibrational effects seen at higher temperatures. An apparatus employing this effect is being used to study the transfer of energy between colliding molecules and the structure of molecular ions.