  A three-dimensional effective theory of the finite-temperature SU(5)+adjoint
Higgs model is constructed using the method of dimensional reduction. The
resulting theory can be used for computer simulations of the GUT phase
transition of the early universe. In this paper the transition is analyzed at
two-loop level in perturbation theory. The structure of the effective theory
does not essentially depend on the matter contents of the original
four-dimensional one. Therefore the analysis of this paper applies also to more
realistic theories.
