  We investigate the evolution of the electroweak phase transition, using a
one-Higgs effective potential that can be regarded as an approximation for the
Minimal Supersymmetric Standard Model. The phase transition occurs in a small
interval around a temperature T_t below the critical one. We calculate this
temperature as a function of the parameters of the potential and of a damping
coefficient related to the viscosity of the plasma. The parameters that are
relevant for baryogenesis, such as the velocity and thickness of the walls of
bubbles and the value of the Higgs field inside them, change significantly in
the range of temperatures where the first-order phase transition can occur.
However, we find that in the likely interval for T_t there is no significant
variation of these parameters. Furthermore, the temperature T_t is in general
not far below the temperature at which bubbles begin to nucleate.
