We have obtained 5-8 mum spectra towards 10 embedded protostars using the S
hort Wavelength Spectrometer on board the Infrared Space Observatory (ISO-S
WS) with the aim of studying the composition of interstellar ices. The spec
tra are dominated by absorption bands at 6.0 mum and 6.85 mum. The observed
peak positions, widths and relative intensities of these bands vary dramat
ically along the different lines of sight. On the basis of comparison with
laboratory spectra, the bulk of the 6.0 mum absorption band is assigned to
amorphous H2O ice. Additional absorption, in this band, is seen toward 5 so
urces on the short wavelength wing, near 5.8 mum, and the long wavelength s
ide near 6.2 mum. We attribute the short wavelength absorption to a combina
tion of formic acid (HCOOH) and formaldehyde (H2CO), while the long wavelen
gth absorption has been assigned to the C-C stretching mode of aromatic str
uctures. From an analysis of the 6.85 mum band, we conclude that this band
is composed of two components: a volatile component centered near 6.75 mum
and a more refractory component at 6.95 mum. From a comparison with various
temperature tracers of the thermal history of interstellar ices, we conclu
de that the two 6.85 mum components are related through thermal processing.
We explore several possible carriers of the 6.85 mum absorption band, but
no satisfactory identification can be made at present. Finally, we discuss
the possible implications for the origin and evolution of interstellar ices
that arise from these new results.