  We present ISO-SWS observations of H2 pure-rotational line emission from the
disks around low and intermediate mass pre-main-sequence stars as well as from
young stars thought to be surrounded by debris disks. We detect `warm' (T ~
100-200 K) H2 gas around many sources, including tentatively the debris-disk
objects. The mass of this warm gas ranges from ~1E-4 Solar mass up to 8E-3
Solar mass, and can constitute a non-negligible fraction of the total disk
mass. Complementary single-dish 12CO 3-2, 13CO 3-2 and 12CO 6-5 observations
have been obtained as well. These transitions probe cooler gas at T ~ 20-80 K.
Most objects show a double-peaked CO emission profile characteristic of a disk
in Keplerian rotation, consistent with interferometer data on the lower-J
lines. The ratios of the 12CO 3-2/ 13CO 3-2 integrated fluxes indicate that
12CO 3-2 is optically thick but that 13CO 3-2 is optically thin or at most
moderately thick. The 13CO 3-2 lines have been used to estimate the cold gas
mass. If a H2/CO conversion factor of 1E4 is adopted, the derived cold gas
masses are factors of 10-200 lower than those deduced from 1.3 millimeter dust
emission assuming a gas/dust ratio of 100,in accordance with previous studies.
The warm gas is typically 1-10 % of the total mass deduced from millimeter
continuum emission, but can increase up to 100% or more for the debris-disk
objects. Thus, residual molecular gas may persist into the debris-disk phase.
No significant evolution in the H2, CO or dust masses is found for stars with
ages in the range of 1E6-1E7 years, although a decrease is found for the older
debris-disk star beta Pictoris. Existing models fail to explain the amount of
warm gas quantitatively.
