Abstract
We study a class of integrable alternating (S_1,S_2) quantum spin chains with critical ground state properties. Our main results are the derivation of a finite set of non-linear integral equations for the thermodynamical properties of the systems and the identification of the thermal currents as conserved currents. These results lead to explicit expressions of the thermal conductivities of the alternating spin chains. Numerical solutions to the integral equations are presented for specific cases of the spins S_1 and S_2.In the low-temperature limit a universal picture evolves where the thermal Drude weight is proportional to temperature T and central charge c.