  We estimate flux ratios of the extrasolar planet XO-2b to its host star XO-2
at 3.6, 4.5, 5.8 and 8.0 micron with IRAC on the Spitzer Space Telescope to be
0.00081 +- 0.00017, 0.00098 +- 0.00020, 0.00167 +- 0.00036 and 0.00133 +-
0.00049, respectively. The fluxes provide tentative evidence for a weak
temperature inversion in the upper atmosphere, the precise nature of which
would need to be confirmed by longer wavelength observations. XO-2b substellar
flux of 0.76 x 10^9 ergs cm^-2 s^-1 lies in the predicted transition region
between atmospheres with and without upper atmospheric temperature inversion.
