The transient absorption spectrum observed in the region 3770–4170 Å during the flash photolysis of thiophene is assigned to the HCCS free radical. Isotope shifts are observed when deuterated thiophene is used and establish the presence of a single H (or D) atom in the carrier. Under high resolution the bands show doublet P and R branches. The bands are tentatively assigned to a 2Π(b)–2Π(b) transition of a linear molecule, though the possibility that the molecule is slightly nonlinear in one of the combining states is not excluded. Rotational assignments are given for the two strongest bands of HCCS, including the O–O band near 4114 Å. The rotational constants for this band are B″0= 0.188 41(6) cm–1 and B′0–B″0=–0.014 399(6) cm–1, where the error limits are 3σ. (See, however, note added in proof.)