  Constraining the astrophysical nature of Ultra-Luminous X-ray (ULX) sources,
which have X-ray luminosities exceeding 10^39 erg/s, has been elusive due to
the optical faintness of any counterparts. With high spectral resolution
observations in the ~10-30 microns wavelength range we have conducted an
experiment to study six ULX sources in the NGC 4485/90 galaxy pair. We have
found that five of the six ULXs, based on mid-infrared spectral diagnostics,
show the characteristic higher ionization features that are found in AGN. The
sixth source, ULX-1, is consistent with being a supernova remnant. The chief
infrared spectral diagnostics used are the ratios of [S III]/[Si II] vs [Ne
III}]/[Ne II]. In two instances fits to the continuum and poly aromatic
hydrocarbons (PAH) features also indicate higher dust temperatures that are
characteristic of accreting sources. Overall, however, we find the continuum is
dominated by stellar processes, and the best diagnostic features are the
emission lines. High spectral resolution studies in the mid-infrared thus
appear to show great promise for determining the astrophysical nature of ULXs.
