  The phase diagram of two-color QCD as a function of temperature and baryon
chemical potential is considered. Using a low-energy chiral Lagrangian based on
the symmetries of the microscopic theory, we determine, at the one-loop level,
the temperature dependence of the critical chemical potential for diquark
condensation and the temperature dependence of the diquark condensate and
baryon density. The prediction for the temperature dependence of the critical
chemical potential is consistent with the one obtained for a dilute Bose gas.
The associated phase transition is shown to be of second order for low
temperatures and first order at higher temperatures. The tricritical point at
which the second order phase transition ends is determined. The results are
carried over to QCD with quarks in the adjoint representation and to ordinary
QCD at a non-zero chemical potential for isospin.
