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 \sim 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 .
Other studies have been made of the ’ warm absorber ’ phenomenon but this paper contains the first consideration of the importance of the covering-fraction of the ionized gas and a direct comparison between models of attenuation by ionized versus neutral material .

We find the X-ray ionization parameter for the ionized material is strongly peaked at U _ { X } \sim 0.1 .
The column densities of ionized material are typically in the range N _ { H,z } \sim 10 ^ { 21 } – 10 ^ { 23 } { cm } ^ { -2 } , although highly ionized ( and hence psuedo-transparent ) column densities up to 10 ^ { 24 } { cm } ^ { -2 } can not 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 .
Allowing a fraction of the continuum to be observed without attenuation also improves the fit to many sources , and is definitely required in the case of NGC 4151 .
A deficit of counts is observed at \sim 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

We discuss these results in the context of previous studies and briefly explore the implications in other wavebands .