Optothermal infrared spectroscopy has been used to study the vibrational modes of N2O-C2H4 that correlate with vg of the ethylene monomer and nu(1) + nu(3) in the N2O subunit. The results show that the N2O lies above the plane of the ethylene and in a rigid picture has its axis parallel to the C=C bond. However, both bands show tunneling splittings that indicate that the two molecules rotate with respect to one another about the axis through their centers-of-mass. Ab initio calculations are in agreement with this, indicating that this is the only feasible tunneling path. The tunneling splittings are used in conjunction with a single axis, two rotor tunneling model to estimate the height of the barrier to internal rotation.