Cardelli continues his study of interstellar gas-phase abundances using the Goddard High Resolution Spectrograph aboard Hubble Space Telescope, concentrating on data obtained with very high signal-to-noise. This work includes the detection and analysis of weak absorption lines from low abundance heavy element atomic species, including Ga, Ge, As, Se, Kr, Cd, Sn, Tl, and Pb. These species are particularly interesting because they are created by nucleosynthetic pathways which are uniquely different from those that create the more traditionally studied lighter elements. As such, they offer an opportunity to study the impact of stellar evolution on the ISM and the efficiency with which this processed material is mixed in the galaxy. An example of a project nearing completion is a study of the interstellar abundance distribution of atomic Krypton, a noble gas element that is not expected to be incorporated into dust. As such, the abundance of Kr should reflect the cosmic (i.e., solar system) value. However, our results indicate that Kr is underabundant relative to the solar system by nearly a factor 2. This supports other recent results that suggest that the elemental abundances in the solar system are anomalously high compared to the relatively nearby interstellar medium, a conclusion supported by the abundance of some elements observed in B-stars. Work on ISM abundances is supported in part by NASA grants NAGW-4607 and HST-GO-5473.01-93A.