The significant structures procedure of liquids has been used to calculate the thermodynamic properties of solid C2H4. Two degeneracy terms were used to describe the behavior in the vicinities of the two phase transitions. The calculated entropy and specific heat apee well with experimental results from a few kelvins to the melting point. Less 1969). Two transitions at 27.0 and 22.1 K were observed in solid heavy methane (1) indicating that, in this material, there exist three solid structures. X-ray measurements revealed a face-centered cubic structure of the molecules (8, 9), except for the low-temperature phase of C2H4, which is face-centered tetragonal (25). Measurements of the nuclear magnetic susceptibility indicated that partial conversion between the three nuclear spin species occurs at 4.2 K (24). Based on all these measurements, the phase transitions in solid methanes are believed to arise from orientational structure change. The theory of these transitions was first studied with statistical mechanics by James and Keenan (26) and has been developed into a quantum-mechanical theory by Yamamoto et al. (27) (refer to this reference for further references in this series up to 1977). The theory of James and Keenan is a classical molecular field theory assuming that the octupole-octupole interaction is the dominant intermolecular force. The three phases in C2H4 are the tetragonal phase III (ferrorotational), the octahedral phase II (antiferrorotational), and the disordered phase I, with a first-order transition followed by a second-order transition. A coherent neutron scattering study (14) confirmed the phase I and phase II structures but ruled out the phase III model proposed by James and Keenan and left the structure of phase III an unsettled issue. The quantum mechanical approach has been successful in many aspects. However, the mathematics involved in such an approach is quite complicated and there are still many problems yet to be solved. In this study, the attempt has been made to use a simple partition function to describe all phases and from it to calculate the thermodynamic properties of solid C2H4 from a few kelvins up to the melting point. The objective of this study is to expand a procedure that has had considerable success in the past in describing liquids to cover this very interesting problem. THEORY In our model a single partition function is written for all three solid phases. The applications of this model are given elsewhere (28,29). For the solid phase of C2H4 molecules, the following structures are considered to be significant. (i) The notations used in Eq. 1 are: frot, rotational partition function; Es, the energy of sublimation; R, gas constant; T, temperature; k, Boltzmann's constant; N, Avogadro's number; q, cluster size; a', change of energy per molecule; X, fraction of molecules with phase III structure; and b', a constant. We have used unity for the partition function for the internal vibrations because the internal vibrational frequencies of C2H4 are quite high [v1 ...