  We present a study of the nature of warm absorbers on the basis of ROSAT
X-ray observations and photoionization calculations carried out with the code
Cloudy. We focus on `non-standard' warm absorbers: (i) the proposed dusty warm
absorbers in IRAS 13349+2438 and IRAS 17020+4544, and possibly in 4C +74.26;
(ii) we explore several scenarios to account for the recently observed peculiar
absorption features around 1.1 keV on the basis of detailed photoionization
models, and apply these to PG 1404+226.
