The electronic spectra of the anions C4O- and C4S- have been observed by absorption spectroscopy in a neon matrix using mass selection. The symmetries of the states are determined on the basis of spectroscopic evidence and ab initio calculations. Two absorption systems of C4O- are assigned to the A2 Sigma+ <--X(2)Pi and B(2)Pi <-- X(2)Pi electronic transitions, with band origins near 875 and 442 nm, respectively. The B(2)Pi <--X(2)Pi system of C4S-, with the origin near 568 nm, has also been observed. After photobleaching, the (3)Sigma (-) <--X(3)Sigma (-) electronic transition of GS is observed around 447 nm.