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This paper presents, for the first time, a complete 2.4-25 mu m spectrum of the dust-embedded young stellar object W33A. The spectrum was obtained with the Short Wavelength Spectrometer of the Infrared Space Observatory at a mean resolving power of similar to 750. The spectrum displays deep H(2)O ice and silicate absorptions centered at 3.0 and 9.7 mu m, respectively, together with absorption features identified with various other molecules in the solid phase. The 2.4-5.0 mu m region of the spectrum is used to investigate the long-standing problem of the H(2)O ice column density toward W33A, by means of the stretching and combination mode features at 3.0 and 4.5 mu m. Although no flux is seen at the center of the 3.0 mu m feature, its central depth may be constrained by fitting assumed profiles to the short- and long-wavelength wings in our spectrum. We deduce that a value of N(H(2)O) = (1.1 +/- 0.3) x 10(19) cm(-2) is consistent with these features, a factor of at least 3 less than predicted by the H(2)O bending mode at 6.0 mu m; the reason for this discrepancy is unclear. We report new results on the abundances of nitrogen-bearing species in the ices toward W33A. Solid NH, is detected for the first time in this line of sight, by means of the inversion-mode feature at 9.0 mu m. The column density is N(NH,)=(1.7 +/- 0.4) x 10(18) cm(-2) implying an abundance of similar to 15% relative to H(2)O, comparable to that recently reported toward the young star NGC 7538 IRS 9. However, we find no convincing evidence for absorptions associated with the C=N stretching mode of nitriles in the 4.4-4.6 mu m region of the spectrum. If nitriles are present in the ices along this line of sight, they must have column density no more than -10(17) cm(-2) or similar to 1% relative to H(2)O. This argues against identification of the deep 4.62 mu m 'XCN' feature with isonitriles, as an implausibly low nitrile to isonitrile abundance ratio (

Data of the Infrared Space Observatory ISO have strongly influenced the current view of interstellar ice chemistry. ISO and ground-based results have confirmed that the most abundant ice species in warm regions close to massive protostars are H2O, CO2, and CH3OH. Ice segregation in those environments reflects the extensive thermal processing of grains over the Lifetime of protostars. We present here a systematic set of laboratory infrared spectra of ice mixtures dominated by H2O, CO2 and CH3OH which have been exposed to thermal and UV irradiation processing. It is shown that the infrared bands of CO2 and of CH3OH are particularly sensitive to the ice composition, temperature and applied UV irradiation. The laboratory data suggest partial crystallization of interstellar ices in the protostellar environment. We present a detailed laboratory study of the CO2 bending mode at 15.2 mu m. The observed multipeak structure of the CO2 bending mode is a result of thermal processing and can not be produced by UV irradiation in the laboratory. Laboratory results show that annealed CO2 ice has a lower stability against UV irradiation than cold amorphous CO2 ice. Annealed ice mixtures containing H2O, CO2 and CH3OH show that the multipeak structure of the CO2 bending mode is not destroyed by UV fluxes of less than or similar to 10(18) photons.cm(-2). Detailed analysis of H2O, CO2 and CH3OH bands show that their profiles can be effectively used to trace the Line of sight conditions and the origin and evolution of the ice composition in dense clouds. The datafiles discussed in this paper can be retrieved from the Leiden observatory database (www.strw.leidenuniv.nl/similar to lab/isodb).