The ν6 and ν10B-type and the ν7 and ν8C-type fundamentals of H 2CCD 2 have been studied at a resolution of ∼0.2 cm -1, and the A-type ν1 fundamental at a resolution of 0.03-0.04 cm -1. From analyses of the rotational structure in these bands, accurate values of the ground state rotational constants A0, B0, and C0 are obtained. The band centers of the very weak B-type bands are accurately located for the first time. For C 2D 4, values of A0, B0, and C0 are determined from the ground state combination differences obtained from the rotational Raman data of Dowling and Stoicheff and the infrared data of Allen and Plyler. For both molecules, the experimental and theoretical inertia defect values are in close agreement. The sets of ground state rotational constants for H 2CCD 2 and C 2D 4 are combined with those for C 2H 4 of a previous study to yield for the ground state geometry of ethylene: r CH0 = 1.085 ± 0.002 Å, r CC0 = 1.339 ± 0.001 Å, αHCH0 = 117°50' ± 15'.