Excess molar enthalpies H-m(E) for (xC(3)H(8) + {1 - x)CH4} have been measured over the entire composition range except at the extremes in x, at the temperatures T = (273.15, 298.15, 323.15, 348.15, 363.15, and 373.15) K and the pressures p = (6, 7.5, 10, 12.5, and 15) MPa. Measurements were also made at p = 5 MPa and T=273.15 K, and at p = (6 and 15) MPa at T = 369.82 K. Thus, measurements have been made at a temperature where both components are liquid (T = 273.15 K), at four temperatures where one component (C3H8) is a liquid while the other (C2H4) is a supercritical fluid T = (298.15, 323.15, 348.15, and 363.15) K, a temperature (T = 369.82 K) at or very near the critical temperature of propane and well above the critical temperature of ethene, and at one temperature where both components are supercritical fluids (T = 373.15 K). At T = 273.15 K and p = 5 MPa, the minimum H-m(E) is -220 J . mol(-1), with the composition corresponding to the minimum skewed towards the ethene-rich mixture. With increasing pressure, H-m(E) becomes less negative, and the minimum changes to a maximum, whose composition switches towards the propane-rich mixture. The result is an s-shaped H-m(E)(x) curve at intermediate pressures and a positive H-m(E)(x) curve at the high pressures. At the higher temperatures, where supercritical fluids are present, the maximum and minimum values of H-m(E) are much larger, and both temperature and pressure have a large effect on H-m(E). Thus H-m(E) changes from a minimum value of approximately -2400 J . mol(-1) with a positive pressure coefficient (partial derivative H-m(E)/partial derivative p)r at p = 6 MPa and T = 298.15 K, to a maximum value of approximately 2900 J . mol(-1) and a negative (partial derivative H-m(E)/partial derivative p)r at p = 6 MPa and T = 373.15 K.