  We consider integrable quantum spin chains with alternating spins (S_1,S_2).
We derive a finite set of non-linear integral equations for the thermodynamics
of these models by use of the quantum transfer matrix approach. Numerical
solutions of the integral equations are provided for quantities like specific
heat, magnetic susceptibility and in the case S_1=S_2 for the thermal Drude
weight. At low temperatures one class of models shows finite magnetization and
the other class presents antiferromagnetic behaviour. The thermal Drude weight
behaves linearly on T at low temperatures and is proportional to the central
charge c of the system. Quite generally, we observe residual entropy for
S_1\neq S_2.
