The thermodynamical properties of the hyperlithiated molecules Li3S and Li4S were investigated by means of Knudsen effusion mass spectrometry and their stability and structure were studied through ab initio molecular orbital calculations. The Li3S and Li4S molecules were found to be stable toward loss of one or two lithium atoms, respectively, to form the octet molecule Li2S. The experimental dissociation energies were D-0(0)(Li2S-Li)=33.1+/-1.6 and D-0(0)(Li2S-2Li)=83.9+/-2.7 kcal/mol. The atomization energies were determined as D-0(0)(Li3S)= 161.3+/-3.8 and D-0(0) (Li4S)= 211.9+/-4.2 kcal/mol. The ionization potential observed for Li3S was 4.4+/-0.2 eV. From the theoretical calculations, the occupancies of nine valence electrons in Li3S (C-3v) and ten valence electrons in Li4S (C-2v) were seen as (5a(1))(2)(3e)(4)(6a(1))(2)(7a(1))(1) and (6a(1))(2)(3b(1))(2)(7a(1))(2)(3b(2))(2)(8a(1))(2), respectively. The singly occupied 7a(1) orbital of Li3S and the highest occupied 8a(1) orbital of Li4S were found to be involved in the formation of a lithium "cage," which should contribute to the thermodynamic stability of these molecules.