  We have studied the implications of the new generation of CMB spectrum space
experiments for our knowledge of the thermal history of the Universe.
  The combination of two experiments with the sensitivity and the frequency
coverage jointly forseen for DIMES and FIRAS II will be able to greatly change
our vision of the capability of the CMB spectrum information to constrain
physical processes at different cosmic ages. The limits on the energy
dissipations at the all cosmic times accessible to CMB spectrum investigations
(z < z_therm) could be improved by about two order of magnitudes and even
dissipation processes with DE/E ~ 10^-6 could be detected and possibly
accurately studied. These results are possible because such levels of accuracy
on a so wide frequency range allow to remove the approximate degeneracy both
between free-free and Bose-Einstein (BE) like distortions and between
Comptonization and BE-like distortions.
  Finally, we discussed the different signatures imprinted on the CMB spectrum
by some late astrophysical and particle decay models recently proposed in the
literature and possibly related to the reionization of the Universe indicated
by WMAP, and compared them with the sensitivity of such classes of CMB space
spectrum experiments.
