  We present interstellar absorption line measurements of the ions S III and Al
III towards six stars using archival Goddard High Resolution Spectrograph data.
The ions Al III and S III trace heavily depleted and non-depleted elements,
respectively, in ionized gas. We use the photoionization code CLOUDY to derive
the ionization correction relating N(Al III)/N(S III) to the gas-phase
abundance [Al/S]_i in the ionized gas. For spectral types considered here, the
corrections are small and independent of the assumed ionization parameter.
Using the results of these photoionization models, we find [Al/S]_i = -1.0 in
the ionized gas towards three disk stars. These values of [Al/S]_i (=[Al/H]_i)
imply that Al-bearing grains are present in the ionized nebulae around these
stars. If the WIM of the Galaxy is photoionized by OB stars, our data for two
halo stars imply [Al/S]_i = -0.4 to -0.5 in the WIM and thus the presence of
dust grains containing Al in this important phase of the ISM. While
photoionization appears to be the most likely origin of the ionization for Al
III and S III, we cannot rule out confusion from the presence of hot,
collisionally ionized gas along two sightlines. We find that [Al/S]_i in the
ionized gas along the six sightlines is anti-correlated with the electron
density and average sightline neutral density. The degree of grain destruction
in the ionized medium of the Galaxy is not much higher than in the warm neutral
medium. The existence of grains in the ionized regions studied here has
important implications for the thermal balance of these regions. (Abstract
Abridged)
