The magnetic entropy variation for the ordered spin-ice Tb2Sn2O7 and the spin-liquid Tb2Ti2O7 measured up to similar to 20 K shows that the crystal-field ground state for both systems is a singlet split from the first excited level by a thermal energy of similar to 2 K. It is the superexchange interactions between the Tb3+ spins which induce the terbium magnetic moments. Our results are analyzed in terms of a tetragonal perturbation for Tb2Ti2O7 and this perturbation plus a molecular field for Tb2Sn2O7.