The J = 1 ← J = 0 and J = 2 ← J = 1 rotational transitions of ruthenium monocarbide, RuC, have been recorded using the separated field pump/probe microwave optical double resonance technique and analyzed to determine the fine and hyperfine parameters for the X 1Σ+ state. The101Ru(I = 5/2) electric quadrupole parameter, eq 0 Q, and nuclear spin-rotation interactionparameter, CeffI , were determined to be 433.19(8) MHz and −0.049(6) MHz, respectively. The equilibrium bond distance, r e, was determined to be 1.605485(2) Å. Hartree-Fock and coupled-cluster calculations were carried out for the properties of the X 1Σ+ state. Electron-correlationeffects are pronounced for all properties studied. It is shown that (a) the moderate scalar-relativistic contribution to eq 0 Q is entirely due to the coupling between scalar-relativistic and electron-correlation effects, (b) the spin-free exact two-component theory in its one-electron variant offers a reliable and efficient treatment of scalar-relativistic effects, and (c) non-relativistic theory performs quite well for the prediction of CelecI , provided that electroncorrelation is treated accurately.