We intend to observe the signature of water ice in the mid-
 and far-infrared towards high mass and low mass embedded
 sources in protostar regions with ISO SWS/LWS and PHOT P.
 Laboratory studies show that the solid water infrared
 features at 3.1 and 45 mu exhibit reversible and irreversible
 temperature effects in band shape and position. Observations of
 these bands towards sources embedded in dense clouds can provide
 constraints on the composition and thermal evolution of the ices
 throughout the dust shell surrounding the sources. The 3.1 mu
 feature will probe the environment relatively close to the star,
 whereas the 45 mu band monitors the very cold dust regions far
 from the radiating source. The 3.1 mu band has been already
 extensively studied by ground-based observations. Observations
 of the 45 mu water ice band will provide the unique opportunity
 to study the chemistry in environments that cannot be probed from
 the ground. ISO is uniquely capable of providing a complete
 spectrum of water ice, including bands obscured by the Earth's
 atmosphere, in particular the 45 mu water ice band.
 Laboratory simulations on water ice will be performed during
 ISO operations to ensure that the full diagnostic value of
 these observations will be achieved.