  Infrared spectroscopic observations toward objects obscured by dense cloud
material show that H$_2$O, CO and, likely, CO$_2$ are important constituents of
interstellar ice mantles. In order to accurately calculate the column densities
of these molecules, it is important to have good measurements of their infrared
band strengths in astrophysical ice analogs. We present the results of
laboratory experiments to determine these band strengths. Improved experimental
methods, relying on simultaneous independent depositions of the molecule to be
studied and of the dominating ice component, have led to accuracies better than
a few percent. Furthermore, the temperature behavior of the infrared band
strengths of CO and H$_2$O are studied. In contrast with previous work, the
strengths of the CO, CO$_2$, and H$_2$O infrared features are found to depend
only weakly on the composition of the ice matrix, and the reversible
temperature dependence of the CO band is found to be weaker than previously
measured for a mixture of CO in H$_2$O.
