  Recent high-resolution observations of metal absorption lines in
high-redshift damped Ly-alpha systems have shown that Al III, a tracer of
moderately-ionized gas, very often has a velocity structure indistinguishable
from that of low-ionization gas. Regions of ionized and neutral hydrogen in
these systems are likely cospatial. The higher-ionization Si IV and C IV
absorption shows a much weaker or non-existent correlation with the low
ionization material, implying that the regions traced by Al III are
photoionized by a soft (stellar) spectrum, by a hard (power law) spectrum with
a very low ionization parameter, or a combination of both. We discuss the
ionization of the damped Ly-alpha systems and use photoionization equilibrium
models to make quantitative estimates of its effects on abundance studies in
these systems. We show that ionization effects may be large enough to account
for the observed dispersion in absolute metal abundances in damped Ly-alpha
systems, causing systematically higher abundances in lower column density
systems. The observed Si^+/Fe^+ and Zn^+/Cr^+ ratios may systematically
overestimate the intrinsic Si/Fe and Zn/Cr ratios, respectively, if ionized gas
is present in these systems, thereby mimicking the effects of alpha-element
enrichment or dust depletion.
