Theoretical studies of diyttrium carbides Y2Cn (n = 1-8) that included both relativistic and electron correlation effects are presented. Consideration of several alternative structures leads to cyclic structures as the groundstate structures. Using the theoretically computed partition functions, thermodynamic data are computed and the experimental Gibbs energy functions are corrected. The computed atomization energies of these species are close to the corrected experimental data. The Y-C bond is strongly ionic due to electron transfer from the yttrium atom to the carbon atom.