We present results of chemical models for a variety of types of regions: sh
own by SWAS observations to contain less O-2 and H2O than previously expect
ed. We identify time-dependent models for which O-2 and H2O abundances meet
the SWAS constraints and for which calculated abundances of other species
are in harmony with measurements made primarily at millimeter wavelengths.
The phases of acceptable composition are transient in these time-dependent
models but are of very substantial length in many models for which CO and N
-2 are assumed to be returned promptly and unaltered to the gas: phase thou
gh other species, except for H-2 and He, freeze-out onto the dust. We also
consider whether the presence of bistability in some steady-state models ca
n account for the SWAS and other observations.