We present observations of CO , ^ { 13 } CO and of H _ { 2 } O in the middle and far-infrared taken with the ISO-SWS and ISO-LWS spectrometers toward two positions in the Galactic Center region ( Sgr A ^ { * } and GCS-3 ) .
Both ice and gas phase molecules are detected .
The ISO data have been complemented with observations of the J=3-2 and J=7-6 lines of CO carried out at the Caltech Submillimeter Observatory .
The ISO and CSO data indicate that the absorbing gas is extremely cold , T _ { K } \simeq 10 K , suggesting that it is located in the dark clouds of the different spiral arms that intersect the line of sight towards the Galactic Center .
From the analysis of the CO absorption we derive ^ { 13 } CO gas phase column densities of 1.1 and 0.7 ~ { } 10 ^ { 17 } cm ^ { -2 } towards Sgr A ^ { * } and GCS-3 respectively .
The H _ { 2 } O gas column density in the direction of Sgr A ^ { * } is \simeq 2 ~ { } 10 ^ { 16 } cm ^ { -2 } .
The derived CO/ { H _ { 2 } O } and gas/solid abundance ratios corresponding to these cold clouds are remarkably similar along the two lines of sight .
We find that nearly all the CO is in the gas phase , while the { H _ { 2 } O } is almost entirely frozen onto the surfaces of cold dust grains .
Finally , the N _ { gas + ice } ( CO ) /N _ { gas + ice } ( H _ { 2 } O ) abundance ratio is \simeq 5 implying that H _ { 2 } O formation processes are highly efficient .