The first optical spectroscopic investigation of MoC has revealed a complicated vibronic spectrum consisting of about 35 bands between 17 700 and 24 000 cm-l. Analysis has shown the ground state to be the Omega = 0(+) spin-orbit component of a (3)Sigma(-) State that derives from a 10 sigma(2)11 sigma(2)5 pi(4)2 delta(2) configuration. The X (3)Sigma(0+)(-) rotational constant for (MoC)-Mo-98-C-12 was determined to be B-0 = 0.553 640 +/- 0.000 055 cm(-1), giving r(0) = 1.687 719 +/- 0.000 084 Angstrom. Consideration of spin-uncoupling effects in the X (3)Sigma(-) State requires that this value be revised to r(0) = 1.6760 Angstrom, which represents our best estimate of the true Mo-C bond length. Spectroscopic constants were also extracted for six other major isotopic modifications of MoC in this mass resolved experiment. All rotationally resolved transitions were found to originate from the ground state and terminate in electronic states with Omega = 1. An attempt is made to classify the observed transitions into band systems, to rationalize the complexity of the spectrum, and to understand the bonding from a molecular orbital point of view.