Excess molar enthalpies H-m(E) for {xC(2)H(6) + (1 - x)C2H4} have been measured over the entire composition range at the temperatures T = (273.15, 298.15, 323.15, 348.15, and 363.15) K and the pressures p = (5, 7.5, 10, 12.5, and 15) MPa. Thus, measurements were made at a temperature where both components are liquid (T = 273.15 K), at a temperature where one component (C2H6) is a liquid while the other (C2H4) is a supercritical fluid (T = 298.15 K), and at three temperatures where both components are supercritical fluids {T = (323.15, 348.15, and 363.15) K}. At T = 273.15 K, a temperature near the critical temperature of ethene, H-m(E)(x) is positive with a large positive pressure coefficient. At T = 298.15 K and p = 5 MPa, a temperature and pressure near, but below, the critical locus, H-m(E)(x) is S shaped, changing from large positive values at low x to negative values at high x. At each of the three higher temperatures, maxima and minima in H-m(E) are obtained at intermediate pressures. A corresponding-states method correctly predicts the pressure and temperature effects on H-m(E) in both the subcritical and supercritical (p, T) regions. At high pressures, H-m(E)(x = 0.5) converges at all the temperatures to a value of approximately 100 J . mol(-1). A (quadrupole + quadrupole) interaction is probably a major contribution to H-m(E) for this system, since ethane and ethene have quadrupole moments of opposite sign.