Laser ionization of AnC4 alloys (An = Th, U) yielded gas-phase molecular thorium and uranium carbide cluster cations of composition An mCn+, with m = 1, n = 2-14, and m = 2, n = 3-18, as detected by Fourier transform ion-cyclotron-resonance mass spectrometry. In the case of thorium, ThmCn+ cluster ions with m = 3-13 and n = 5-30 were also produced, with an intriguing high intensity of Th13Cn+ cations. The AnC 13+ ions also exhibited an unexpectedly high abundance, in contrast to the gradual decrease in the intensity of other AnCn + ions with increasing values of n. High abundances of AnC 2+ and AnC4+ ions are consistent with enhanced stability due to strong metal-C2 bonds. Among the most abundant bimetallic ions was Th2C3+ for thorium; in contrast, U2C4+ was the most intense bimetallic for uranium, with essentially no U2C 3+ appearing. Density functional theory computations were performed to illuminate this distinction between thorium and uranium. The computational results revealed structural and energetic disparities for the An2C3+ and An2C4 + cluster ions, which elucidate the observed differing abundances of the bimetallic carbide ions. Particularly noteworthy is that the Th atoms are essentially equivalent in Th2C3+, whereas there is a large asymmetry between the U atoms in U2C3 +.