  The longitudinal thermopower of ultra-high mobility two-dimensional electrons
has been measured at both zero magnetic field and at high fields in the
compressible metallic state at filling factor $\nu = 3/2$ and the
incompressible fractional quantized Hall state at $\nu = 5/2$. At zero field
our results demonstrate that the thermopower is dominated by electron diffusion
for temperatures below about $T = 150$ mK. A diffusion dominated thermopower is
also observed at $\nu = 3/2$ and allows us to extract an estimate of the
composite fermion effective mass. At $\nu = 5/2$ both the temperature and
magnetic field dependence of the observed thermopower clearly signal the
presence of the energy gap of this fractional quantized Hall state. We find
that the thermopower in the vicinity of $\nu = 5/2$ exceeds that recently
predicted under the assumption that the entropy of the 2D system is dominated
by non-abelian quasiparticle exchange statistics.
