Ethylene is an exceptional example of a stable closed-shell singlet molecule with a low-lying triplet state of very different symmetry. Triplet C2H4 (the (a) over tilde (3)A(1) state), which has a twisted D-2d geometry, is studied herein with high-level theoretical methods, namely, CCSD(T) (coupled cluster theory with single, double, and perturbative triple excitations) and Dunning's correlation-consistent quadruple-zeta basis set (cc-pVQZ). Geometric parameters, including equilibrium (r(e)) and vibrationally corrected (r(g)) values, are reported for C2H4, C2D4, and (C2H4)-C-13. Harmonic and anharmonic vibrational frequencies are also predicted using second-order vibrational perturbation theory (VPT2). Challenges encountered for the wagging vibrational features are discussed.