Hyperfine splitting constants (hfs) of the (X) over tilde (2)A＇ electronic ground state of the thioformyl radical (HCS) have been determined at the coupled-cluster level with single, double, and perturbatively applied connected triple excitations [CCSD(T)] using 39 basis sets. Variation of the CCSD(T) hyperfine splittings with basis set was ascertained using a fixed geometry, optimized at the CCSD(T) level with Dunning＇s correlation-consistent polarized valence quadruple-zeta basis set (cc-pVQZ). Pople basis sets, 6-311G++(2d,2p) and 6-311G++(3df,3pd), give H-1 isotropic coupling constants (H-1 A(iso)) in good agreement with the experimental vibrationally averaged value of 127.4 MHz, deviating by 5.5 and 9.3 MHz, respectively. Dunning＇s valence correlation-consistent basis sets (cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, aug-cc-pVTZ, cc-pVQZ, aug-cc-pVQZ) deviate 7.4 MHz (aug-cc-pVQZ) to 14.9 MHz (cc-pVDZ) from the experimental value. The correlation-consistent core valence analogues of these sets give very similar values with deviations from experiment of 7.4 MHz (cc-pCVQZ) to 14.2 MHz (cc-pCVDZ). A direct comparison with the vibrationally averaged experimental value is not precisely possible since the hyperfine splittings are strongly geometry dependent and all theoretical predictions refer to the equilibrium geometry. Small Pople basis sets (3-12G, 6-31G, and 6-311G) give the worst results, deviating by 49.5, 34.1, and 31.8 MHz, respectively. All CCSD(T) H-1 A(iso) values fall below the experimental value. The C-13 and S-33 hyperfine splittings are not known experimentally, but the equilibrium values are predicted here to be 274.7 MHz (C-13) and 21.7 MHz (S-33) at the cc-pCVQZ CCSD(T) level of theory. Significantly different values are predicted by density functional theory (DFT) for the C-13 and S-33 hyperfine splittings. (C) 2000 American Institute of Physics. [S0021-9606(00)31613-0].