  We study the spontaneous CP breaking at finite temperature in the Higgs
sector in the Minimal Supersymmetric Standard Model with a gauge singlet. We
consider the contribution of the standard model particles and that of stops,
charginos, neutralinos, charged and neutral Higgs boson to the one-loop
effective potential. Plasma effects for all bosons are also included. Assuming
CP conservation at zero temperature, so that experimental constraints coming
from, {\it e.g.}, the electric dipole moment of the neutron are avoided, and
the electroweak phase transition to be of the first order and proceeding via
bubble nucleation, we show that spontaneous CP breaking cannot occur inside the
bubble mainly due to large effects coming from the Higgs sector. However,
spontaneous CP breaking can be present in the region of interest for the
generation of the baryon asymmetry, namely inside the bubble wall. The
important presence of very tiny explicit CP violating phases is also commented.
