Vibronic band systems for the HC4S and DC4S radicals were observed in supersonic jets for the first time by laser induced fluorescence (LIF) spectroscopy. The rotational structures of the bands were resolved by high-resolution scans. It was found that all the observed bands are (2)Pi(3/2)-(2)Pi(3/2), transitions. Band origins and effective rotational constants of the origin bands are determined to be T-0 = 19980.687(l) and B'(eff) = 0.046580(4) cm(-1) for HC4S, T-0 = 20034.8748(9) and B'(eff) = 0.044294(3) cm(-1) for DC4S. Ab initio calculations were carried out to interpret the vibronic transitions. (C) 2002 Elsevier Science B.V. All rights reserved.