  We present preliminary results from spectral synthesis models of old stellar
populations for the spectral range 912-4000 A with ~ 10A resolution, which can
be used to investigate the UVX phenomenon and to assess ages and abundances.
Model spectra incorporating extreme horizontal branch (EHB) and Post-Asymptotic
Giant Branch (P-AGB) populations give good matches to the far-UV spectra of
galaxies. These models indicate an EHB fraction which is < 10% of the total HB
population in all but the most extreme examples of the UVX phenomenon, where
the EHB fraction is still <= 20. Once the hot component that gives rise to the
UVX phenomenon is accounted for, the mid-UV wavelength range (2200 < lambda <
3300 A) provides information about the age and metallicity of the underlying
stellar population. The flux in this spectral range arises mainly from stars
close to the main sequence turnoff. We compare models with the spectrum of M31
and discuss UV features which should be useful as population diagnostics.
