The rate coefficients for the forward and reverse proton-transfer reactions C2H4 + H3O+ reversible arrow C2H5+ + H2O are studied with respect to independent varied neutral molecule and ion temperatures. The measurements are performed using a coaxial molecular beam radio frequency ring electrode ion trap at trap temperatures down to 23 K and beam temperatures up to 450 K. The temperature-dependent rate coefficients suggest that in this temperature window, the reaction proceeds through a statistically equilibrated complex. In order to explain the observed rate coefficients, a new type of reaction temperature was defined in these studies that considered collisional and internal (rotational and vibrational) degrees of freedom of both H3O+ and C2H4. The enthalpy and entropy of the equilibrium reaction deduced from a Van't Hoff plot are Delta H = (5.1 +/- 0.5) kJ.mol(-1) and Delta S = (-15.0 +/- 0.9) J.mol(-1).K-1, respectively.