A new thermodynamic property formulation based upon a fundamental equation explicit in Helmholtz energy of the form A = A(p, T) for ethylene from the freezing line to 450 K at pressures to 260 MPa is presented. A vapor pressure equation, equations for the saturated liquid and vapor densities as functions of temperature, and an equation for the ideal-gas heat capacity are also included. The fundamental equation was selected from a comprehensive function of 100 terms on the basis of a statistical analysis of the quality of the fit. The coefficients of the fundamental equation were determined by a weighted least-squares fit to selected P-p-T data, saturated liquid and saturated vapor density data to define the phase equilibrium criteria for coexistence, Cv data, velocity of sound data, and second virial coefficients. The fundamental equation and the derivative functions for calculating internal energy, enthalpy, entropy, isochoric heat capacity (Cv), isobaric heat capacity (Cp), and velocity of sound are included. The fundamental equation reported here may be used to calculate pressures and densities with an uncertainty of +0.1%, heat capacities within + 3 %, and velocity of sound values within + 1%, except in the region near the critical point. The fundamental equation is not intended for use near the critical point. This formulation is proposed as part of a new international standard for thermodynamic properties of ethylene.