  We present observations of Taurus-Auriga Class I/II protostars obtained with
the Spitzer InfraRed Spectrograph. Detailed spectral fits to the 6 and 15
micron features are made, using publicly-available laboratory data, to
constrain the molecular composition, abundances, and levels of thermal
processing along the lines of sight. We provide an inventory of the molecular
environments observed, which have an average composition dominated by water ice
with ~12% CO_2 (abundance relative to H_2O), >~2-9% CH_3OH, <~14% NH_3, ~4%
CH_4, ~2% H_2CO, ~0.6% HCOOH, and ~0.5% SO_2. We find CO_2/H_2O ratios nearly
equivalent to those observed in cold clouds and lines of sight toward the
galactic center. The unidentified 6.8 micron profiles vary from source to
source, and it is shown to be likely that even combinations of the most common
candidates (NH_4+ and CH_3OH) are inadequate to explain the feature fully. We
discuss correlations among SED spectral indices, abundance ratios, and
thermally-processed ice fractions and their implications for CO_2 formation and
evolution. Comparison of our spectral fits with cold molecular cloud
sight-lines indicates abundant prestellar ice environments made even richer by
the radiative effects of protostars. Our results add additional constraints and
a finer level of detail to current full-scale models of protostellar and
protoplanetary systems.
