  (Abridged) We present the results from a detailed analysis of the 0.6 - 10
keV spectra of 23 ASCA observations of 18 objects. We find that in most cases
the underlying continuum can be well-represented by a powerlaw with a photon
index Gamma approx 2. However we find strong evidence for photoionized gas in
the line-of-sight to 13/18 objects. We present detailed modelling of this gas
based upon the ION photoionization code. We find the X-ray ionization parameter
for the ionized material is strongly peaked at Ux approx 0.1. The column
densities of ionized material are typically in the range N_{H,z} approx
10^{21}--10^{23} cm^{-2}, although highly ionized column densities up to
10^{24} cm^{-2} cannot be excluded in some cases. We also investigate the
importance of the emission-spectrum from the ionized gas, finding that it
significantly improves the fit to many sources with an intensity consistent
with material subtending a large solid angle at the central source. A deficit
of counts is observed at approx 1 keV in the sources exhibiting the strongest
absorption features. We suggest this is likely to be the signature of a second
zone of (more highly) ionized gas, which might have been seen previously in the
deep Fe K-shell edges observed in some Ginga observations. We find evidence
that the ionized material in NGC 3227 and MCG-6-30-15 contains embedded dust,
whilst there is no such evidence in the other sources
