Laser-ablated thorium and uranium atoms have been co-deposited at 4 K with hydrogen sulfide in excess noble gas matrixes. The major dihydride sulfide reaction products were observed for each actinide and identified on the basis of S-34 and D isotopic substitution. These assignments were confirmed by frequency and structure calculations using density functional theory with the B3LYP and PW91 exchange-correlation functionals and the CCSD(T) method for the pyramidal H2ThS ((1)A') and H2US ((3)A″) molecules. The lowest three spin states of triplet H2US are calculated to be within 3 kcal/mol using all three methods, just as in H2UO. The major products are compared with the oxygen analogues H2ThO and H2UO, and the sulfides have 71-85 cm(-1) higher hydrogen-actinide stretching frequencies. The actinide-hydrogen bonding appears to be enhanced in the actinide sulfides through back-bonding of a S 3p electron pair to a vacant 6d orbital, which is delocalized over the H atoms. This unique covalent bond is favored by the inductive effect of the hydride substituents, the pyramidal structures, and the lower electronegativity of sulfur. Sulfur back-bonding gives polarized triple bond character to the US and ThS bonds and enhanced metal hydride bonding in H2ThS and H2US.