  A development of perturbations of number densities of ions and electrons
during recombination epoch is analysed. The equations for relative
perturbations of ionization fractions were derived from the system of equations
for accurate computation of the ionization history of the early Universe given
by Seager et al. (1999,2000). It is shown that strong dependence of ionization
and recombination rates on the density and temperature of plasma provides the
significant deviations of amplitudes of ionization fractions relative
perturbations from ones of baryon matter density adiabatic perturbations. Such
deviations are most prominent for cosmological adiabatic perturbations of
scales larger than sound horizon at recombination epoch. The amplitudes of
relative perturbations of number densities of electrons and protons at last
scattering surface exceed by factor of $\simeq$5 the amplitude of relative
perturbation of baryons total number density, for helium ions this ratio
reaches the value of $\simeq$18. For subhorizon cosmological perturbations
these ratios appear to be essentially lesser and depend on oscillation phase at
the moment of decoupling. These perturbations of number densities of ions and
electrons at recombination epoch do not contribute to the intrinsic plasma
temperature fluctuations but cause the ''corrugation'' of last scattering
surface in optical depth, $\delta z_{dec}/(z_{dec}+1)\approx -\delta_b/3$, at
scales larger than sound horizon. It may result into noticeable changes of
precalculated values of CMB polarization pattern at several degrees angular
scales.
