Absorption spectra from 4000 to 1200 cm -1 of amorphous solid water and polycrystalline ice I c have been measured between 10 K and 140 K. Warm up and recooling of an H 2O sample prepared at 10 K gives rise to both irreversible and reversible changes in the peak frequency, band width, and peak height as well as the integrated intensity of the OH stretching band. These spectral effects are related to structural differences. The structure of amorphous solid water also depends on deposition conditions. The optical constants of amorphous so water are determined at 10 K and 80 K from a Kramers-Kronig analysis of the transmission spectra taking into account reflection and interference losses. The astrophysical implication of the temperature dependence of peak frequency and band width of the 3250 cm -1 band in amorphous solid water is discussed briefly.