  Resent observations of a number of galaxy clusters using the
Sunyaev-Zel'dovich effect indicate that about 1/3 of baryonic mass is missing
from the hot intracluster medium (ICM), which is significantly larger than the
fraction of stars and cool gas, which account for only about 10%. Here we
address the question whether the remaining $22\pm10%$ can be accounted for by
thermal evaporation of gas from clusters. We have found that evaporation can
occur only from the cluster ``surface'', $r\sim r_{\rm vir}$, and not from it's
interior. We evaluated particle diffusion through the magnetized ICM for
several scenarios of ISM turbulence and found that diffusivity is suppressed by
at least a factor of 100 or more, compared to the Spitzer value. Thus, only
particles from radii $r\ga0.9r_{\rm vir}$ can evaporate. Diffusion of particles
from inside the cluster, $r\la0.9r_{\rm vir}$, takes longer than the Hubble
time. This lowers the cluster-averaged fraction of the evaporated hot gas to
few percent or less. However, if the missing hot component {\it is indeed} due
to evaporation, this strongly constrains the magnetic field structure in the
cluster envelope, namely either (i) the gas is completely unmagnetized
($B\le10^{-21}$ gauss) in the cluster halo or (ii) the magnetic fields in the
ICM are rather homogeneous and non-turbulent.
