  It is well known that recombination radiation of primordial hydrogen-helium
plasma leads to the distortions of the planckian spectrum shape of the cosmic
microwave background radiation (CMB). We discuss the thermal Sunyaev-Zeldovich
(SZ) effect with taking into account primordial recombination radiation (PRR).
Since in the thermal SZ effect the redistribution of the photons depends on the
derivatives of the spectrum, the value of relative correction to SZ effect due
to PRR significantly higher than relative corrections due to PRR in the initial
spectrum.
  Calculations of corrections to the thermal SZ effect due to PRR show that
depending on the cluster parameters:
  1) in the range of frequencies $\nu$ = 0.3 - 700 GHz where the cosmic
microwave background (CMB) dominates and spectrum is very close to the
planckian one the relative corrections due to PRR have an order of $10^{-9} -
10^{-6}$ of "pure" SZ effect (i.e. SZ effect for pure planckian spectrum). The
difference of intensities of PRR coming from different directions (through
intracluster and near intercluster medium) reaches values up to 22 mJy/ster at
$\nu\simeq 337$ GHz (maximum in considered range).
  2) In the range of frequencies $\nu$ = 700 - 5000 GHz where cosmic infrared
background (CIB) becomes significant or even dominates the relative corrections
due to PRR can reach $10^{-8}$ - $10^{-5}$ of "pure" SZ effect. Corresponding
intensity difference reaches values up to 25 mJy/ster (at $\nu\simeq 1700$
GHz).
  In addition we suggest a modification of the method of electron gas
temperature determination using corrections of the SZ effect due to PRR. Such
modification allows one to simplify the determination of the cluster electron
gas temperature in comparison with known methods.
