Laser radiation (XeCl laser, 308-nanometer wavelength) focused into a cell containing Mo(CO), vapor produced ultrafine particles in the extended waist of the laser beam. Negative ion mass spectrometry revealed molybdenum carbide cluster ions with a stoichiometry MonC4n (n = 1 to 4). The MonC4n- (n = 2 to 4) ions are completely unreactive with NH3, H2O, and O2, suggesting structures in which the molybdenum atoms are unavailable for coordination to additional ligands. Collision-induced dissociation studies of these anions show the loss of MoC4 units as the main fragmentation pathway. This observation, together with the lack of addition reactions, provides a basis for structures in which a planar cluster of two, three, or four molybdenum atoms is surrounded by, and bonded to, carbon dimers.