Band structure calculations show that substitutional lithium in bismuth can form sharp density of states peaks close to the Fermi energy, suggesting the presence of a resonant level, whereas interstitial lithium is expected to act as a simple donor impurity. Experimentally, polycrystalline elemental bismuth samples doped with 0.3%, 0.7%, 1.5%, 3%, 14% (a eutectic composition between Bi and the tetragonal intermetallic compound BiLi), and 20% lithium were prepared. Single-crystal and polycrystalline Bi88Sb12 samples were also doped with lithium. The thermal and thermoelectric transport data of those compounds are reported. The data are consistent with lithium being a donor, leading to the conclusion that it is an interstitial impurity. The thermal conductivity of Bi decreases with Li content at low concentrations, but that of the Bi/BiLi eutectic composition shows the highest value.