  The composite operator formalism is applied to QCD at finite temperature to
calculate the masses of scalar and pseudoscalar mesons. In particular the ratio
of the sigma mass to the pion mass is an interesting measure of the degree of
chiral symmetry breaking at different temperatures. We calculate the
temperature T* at which M_sigma(T) < 2M_pi(T), above which the sigma partial
width into two pions vanishes. We find T*=0.95T_c (where T_c is the critical
temperature for the chiral phase transition), within the full effective
potential given by the formalism. We find that an expansion a-la Landau of the
effective potential around the critical point in the limit of small quark mass
provides for a very good determination of T*.
