  We present an independent estimate of the interstellar extinction law for the
Spitzer IRAC bands as well as a first attempt at extending the law to the
24micron MIPS band. The source data for these measurements are observations of
five nearby star-forming regions: the Orion A cloud, NGC 2068/71, NGC 2024/23,
Serpens and Ophiuchus. Color excess ratios E(H-Ks)/E(Ks-[lambda]) were measured
for stars without infrared excess dust emission from circumstellar
disks/envelopes.
  For four of these five regions, the extinction laws are similar at all
wavelengths and differ systematically from a previous determination of the
extinction law, which was dominated by the diffuse ISM, derived for the IRAC
bands. This difference could be due to the difference in the dust properties of
the dense molecular clouds observed here and those of the diffuse ISM. The
extinction law at longer wavelengths toward the Ophiuchus region lies between
that to the other four regions studied here and that for the ISM. In addition,
we extended our extinction law determination to 24micron for Serpens and NGC
2068/71 using Spitzer MIPS data. We compare these results against several ISO
extinction law determinations, although in each case there are assumptions
which make absolute comparison uncertain. However, our work confirms a
relatively flatter extinction curve from 4 - 8micron than the previously
assumed standard, as noted by all of these recent studies. The extinction law
at 24micron is consistent with previous measurements and models, although there
are relatively large uncertainties.
