We present a study of gas-phase H _ { 2 } O and CO _ { 2 } toward a sample of 14 massive protostars with the ISO-SWS .
Modeling of the H _ { 2 } O spectra using a homogeneous model with a constant excitation temperature T _ { ex } shows that the H _ { 2 } O abundances increase with temperature , up to a few times 10 ^ { -5 } with respect to H _ { 2 } for the hottest sources ( T _ { ex } \sim 500 K ) .
This is still a factor of 10 lower than the H _ { 2 } O ice abundances observed toward cold sources in which evaporation is not significant ( Keane et al .
2001 ) .
Gas-phase CO _ { 2 } is not abundant in our sources .
The abundances are nearly constant for T _ { ex } \ga 100 K at a value of a few times 10 ^ { -7 } , much lower than the solid-state abundances of \sim 1–3 \times 10 ^ { -6 } ( Gerakines et al .
1999 ) .
For both H _ { 2 } O and CO _ { 2 } the gas/solid ratio increases with temperature , but the increase is much stronger for H _ { 2 } O than for CO _ { 2 } , suggesting a different type of chemistry .
In addition to the homogeneous models , a power law model has been developed for one of our sources , based on the physical structure of this region as determined from submillimeter data by van der Tak et al .
( 1999 ) .
The resulting H _ { 2 } O model spectrum gives a good fit to the data .