  We present spectra obtained with the Infrared Spectrograph (IRS) on the
Spitzer Space Telescope of 33 K giants and 20 A dwarfs to assess their
suitability as spectrophotometric standard stars. The K giants confirm previous
findings that the strength of the SiO absorption band at 8 um increases for
both later optical spectral classes and redder (B-V)_0 colors, but with
considerable scatter. For K giants, the synthetic spectra underpredict the
strengths of the molecular bands from SiO and OH. For these reasons, the
assumed true spectra for K giants should be based on neither the assumption
that molecular band strengths in the infrared can be predicted accurately from
optical spectral class or color nor synthetric spectra. The OH bands in K
giants grow stronger with cooler stellar temperatures, and they are stronger
than predicted by synthetic spectra. As a group, A dwarfs are better behaved
and more predictable than the K giants, but they are more likely to show red
excesses from debris disks. No suitable A dwarfs were located in parts of the
sky continuously observable from Spitzer, and with previous means of estimating
the true spectra of K giants ruled out, it was necessary to use models of A
dwarfs to calibrate spectra of K giants from observed spectral ratios of the
two groups and then use the calibrated K giants as standards for the full
database of infrared spectra from Spitzer. We also describe a lingering
artifact that affects the spectra of faint blue sources at 24 um.
