Since hot 100,000-1,000,000K gas in stars radiates predominantly at EUV and soft X-ray wavelengths, observations in
these bands provide important diagnostics of the physical conditions in hot photospheres, stellar coronae and stellar
winds. However, such studies are only able to examine the bulk of material, without being able to separate out the
several gas components present. Radial velocity diagnostics have been used frequently in the UV and visible bands to
distinguish different emission or absorption components in stellar spectra. Now, developments in grating and instrument
technology provide a first opportunity to extend this technique into the EUV. Based on capabilities of the improved JPEX
spectrometer (reported elsewhere in this volume), this paper reports on the key science that might be carried out
with such an instrument; both as a sounding rocket payload and longer duration mission.